Feature/forced bucketing (#145)

* initial implementation of forced bucketing calling into java sdk 1.8.0 released today.

* add forcedBucketing tracking tests

* confirm that event handler sent event with proper variation id, fix typo

* setup mock bucketer to alway return var_1. use it to test against forced variation to var_2

* add one more forced variation test

Author: thomaszurkan-optimizely