Added feature variables (#170)

* - Updated Java sdk version from 1.8 to 2.0.0-beta2 (#9)

* - Updated Java sdk version from 1.8 to 2.0.0-beta2
- Removed
   getVariableString
   getVariableBoolean
   getVariableInteger
   getVariableDouble
-Remove track with revenue as a parameter. Pass the revenue value as an event tag instead
   track(String, String, long)
   track(String, String, Map<String, String>, long)

* added multidex enabled support

* make common Constant of $multidex_ver to use in all gradle

* Added Feature variables and its testcases (#12)

* Added Feature Variables in optimizely client

* Updated Client Test cases of feature Flags

* Updated test cases of all Feature variables

* Upgraded Java sdk (#13)

* Excluded  group: 'com.google.code.findbugs' from dependency of com.optimizely.ab:core-api
removed Multidex enabling from sdk and test app

* Updated Test unit tests

* Feature variable changes asked (#22)

* parameterized optimizelyClientTest.java

* Updated test cases of feature variables and updated validProjectconfigv4

* Code refactoring

* Feature variable changes asked

* parameterized optimizelyClientTest.java

* Updated test cases of feature variables and updated validProjectconfigv4

* Code refactoring

* Refactored code

Author: mnoman09