[RocksJava] Slice / DirectSlice improvements

Summary:
- AssertionError when initialized with Non-Direct Buffer
- Tests + coverage for DirectSlice
- Slice sigsegv fixes when initializing from String and byte arrays
- Slice Tests

Test Plan: Run tests without source modifications.

Reviewers: yhchiang, adamretter, ankgup87

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D30081
[RocksJava] Incorporate additions for D30081
[RocksJava] Incorporated changes D30081
[RocksJava] JavaDoc correction
[RocksJava] Incorporated changes D30081
Merge pull request #437 from fyrz/RocksJava-SliceTests-Fixes

[RocksJava] Slice / DirectSlice improvements