Implemented StringAppendOperator and unit tests.

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

utilities/merge_operators/string_append/stringappend.h

@@ -0,0 +1,31 @@
+/**
+ * A MergeOperator for rocksdb/leveldb that implements string append.
+ * @author Deon Nicholas (dnicholas@fb.com)
+ * Copyright 2013 Facebook
+ */
+
+#include "leveldb/merge_operator.h"
+#include "leveldb/slice.h"
+
+namespace leveldb {
+
+class StringAppendOperator : public MergeOperator {
+ public:
+
+  StringAppendOperator(char delim_char);    /// Constructor: specify delimiter
+
+  virtual void Merge(const Slice& key,
+                     const Slice* existing_value,
+                     const Slice& value,
+                     std::string* new_value,
+                     Logger* logger) const override;
+
+  virtual const char* Name() const override;
+
+ private:
+ char delim_;         // The delimiter is inserted between elements
+
+};
+
+} // namespace leveldb
+