Titile: a command line shell to read/write data from a leveldb thrift server

Summary: implemented a commond line shell to talk with leveldb thrift server, which is based on state pattern and can be easily extended.

Test Plan: build and run

Reviewers: dhruba, zshao, heyongqiang

Differential Revision: https://reviews.facebook.net/D4713

tools/shell/LeveldbShell.cpp

@@ -0,0 +1,8 @@
+
+
+#include "ShellContext.h"
+
+int main(int argc, char ** argv) {
+  ShellContext c(argc, argv);
+  c.run();
+}