Add nullable from javax.annotation;

Move log4j.properties to test resources, it is only for testing.
For production env, app service should have it's own log4j properties file, or use other logging framework.

Author: Javen

.classpath

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <classpath>
 	<classpathentry kind="src" path="src/main/java"/>
+	<classpathentry kind="src" path="src/test/resources"/>
 	<classpathentry kind="src" path="src/main/resources"/>
 	<classpathentry kind="src" path="src/test/java"/>
 	<classpathentry kind="con" path="org.eclipse.jdt.junit.JUNIT_CONTAINER/4"/>

src/main/java/cn/jpush/api/utils/Nullable.java

@@ -0,0 +1,16 @@
+package cn.jpush.api.utils;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
+/**
+ * Copied from javax.annotation.Nullable
+ */
+@Documented
+//@TypeQualifierNickname
+//@Nonnull(when = When.UNKNOWN)
+@Retention(RetentionPolicy.RUNTIME)
+public @interface Nullable {
+
+}

src/main/java/cn/jpush/api/utils/Preconditions.java

@@ -13,9 +13,9 @@
  */
 package cn.jpush.api.utils;
 
-import com.sun.istack.internal.Nullable;
-
 /**
+ * Copied from Google Guava.
+ * 
  * Static convenience methods that help a method or constructor check whether it was invoked
  * correctly (whether its <i>preconditions</i> have been met). These methods generally accept a
  * {@code boolean} expression which is expected to be {@code true} (or in the case of {@code