Summary:
Implemented the StringAppendOperator class (subclass of MergeOperator).
Found in utilities/merge_operators/string_append/stringappend.{h,cc}

It is a rocksdb Merge Operator that supports string/list concatenation
 with a configurable delimiter.

The tests are found in .../stringappend_test.cc. It implements a
 map : key -> (list of strings), with core operations Append(list_key,val)
 and Get(list_key).

Test Plan:
1. Navigate to your rocksdb repository
2. Execute: make stringappend_test  (to compile)
3. Execute: ./stringappend_test (to run the tests)
4. Execute: make all check (to test the ENTIRE rocksdb codebase / regression)

Reviewers: haobo, dhruba, zshao

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D10737