initial commit for ABTest

- API (draft)
- SPI (draft)
Merge pull request #24 from unidal/abtest

initial commit for ABTest
integrate group strategy into context manager
Merge pull request #25 from unidal/abtest

integrate group strategy into context manager
define data model
Merge pull request #29 from unidal/abtest

define data model
完成：从abtest.xml读取entity，初始化DefaultABTestEntityManager; DefaultABTestEntityManager的UT。