Allow to specify a pymysql wrapper or subclass

This will allow to implement custom transports (tls transport and not mysql own
starttls equivalent)

example usage:
```python
import socket

import pymysql.connections
from pymysql.constants import CLIENT

class TLSConnection(pymysql.connections.Connection):
    def __init__(self, *args, **kwargs):
        kwargs['defer_connect'] = True

        super(TLSConnection, self).__init__(*args, **kwargs)
        self.ssl = False
        self.client_flag &= ~CLIENT.SSL
        sock = socket.create_connection((self.host, self.port), self.connect_timeout)
        self.host_info = "socket %s:%d" % (self.host, self.port)
        sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)
        sock = self.ctx.wrap_socket(sock, server_hostname=self.host)

        self.connect(sock)

mysql_settings={
    'host': '1.2.3.4',
    'user': 'acme',
    'password': 'hackme',
    'port': 3306,
    'charset': 'utf8mb4',
    'db': '',
    'ssl': {
      'ca': 'root.crt',
      'cert': 'crt',
      'key':  'key',
      'cipher': 'ECDHE-RSA-AES256-GCM-SHA384'
    }
}

from pymysqlreplication import BinLogStreamReader
stream = BinLogStreamReader(connection_settings=mysql_settings, pymysql_wrapper=TLSConnection)

```

Signed-off-by: Arthur Gautier <baloo@gandi.net>

Author: Arthur Gautier

pymysqlreplication/binlogstream.py

@@ -132,7 +132,8 @@ def __init__(self, connection_settings, server_id, resume_stream=False,
                  ignored_events=None, auto_position=None,
                  only_tables=None, only_schemas=None,
                  freeze_schema=False, skip_to_timestamp=None,
-                 report_slave=None, slave_uuid=None):
+                 report_slave=None, slave_uuid=None,
+                 pymysql_wrapper=None):
         """
         Attributes:
             resume_stream: Start for event from position or the latest event of
@@ -183,6 +184,11 @@ def __init__(self, connection_settings, server_id, resume_stream=False,
             self.report_slave = ReportSlave(report_slave)
         self.slave_uuid = slave_uuid
 
+        if pymysql_wrapper:
+            self.pymysql_wrapper = pymysql_wrapper
+        else:
+            self.pymysql_wrapper = pymysql.connect
+
     def close(self):
         if self.__connected_stream:
             self._stream_connection.close()
@@ -198,7 +204,7 @@ def __connect_to_ctl(self):
         self._ctl_connection_settings = dict(self.__connection_settings)
         self._ctl_connection_settings["db"] = "information_schema"
         self._ctl_connection_settings["cursorclass"] = DictCursor
-        self._ctl_connection = pymysql.connect(**self._ctl_connection_settings)
+        self._ctl_connection = self.pymysql_wrapper(**self._ctl_connection_settings)
         self._ctl_connection._get_table_information = self.__get_table_information
         self.__connected_ctl = True
 
@@ -236,7 +242,7 @@ def __connect_to_stream(self):
         # flags (2) BINLOG_DUMP_NON_BLOCK (0 or 1)
         # server_id (4) -- server id of this slave
         # log_file (string.EOF) -- filename of the binlog on the master
-        self._stream_connection = pymysql.connect(**self.__connection_settings)
+        self._stream_connection = self.pymysql_wrapper(**self.__connection_settings)
 
         self.__use_checksum = self.__checksum_enabled()