Don't compile ldb tool into static library

Summary:
This is first step of my effort to reduce size of librocksdb.a for use in mobile.

ldb object files are huge and are ment to be used as a command line tool. I moved them to `tools/` directory and include them only when compiling `ldb`

This diff reduced librocksdb.a from 42MB to 39MB on my mac (not stripped).

Test Plan: ran ldb

Reviewers: dhruba, haobo, sdong, ljin, yhchiang

Reviewed By: yhchiang

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17823

include/rocksdb/ldb_tool.h

@@ -1,18 +0,0 @@
-// Copyright (c) 2013, Facebook, Inc.  All rights reserved.
-// This source code is licensed under the BSD-style license found in the
-// LICENSE file in the root directory of this source tree. An additional grant
-// of patent rights can be found in the PATENTS file in the same directory.
-#ifndef STORAGE_ROCKSDB_INCLUDE_LDB_TOOL_H
-#define STORAGE_ROCKSDB_INCLUDE_LDB_TOOL_H
-#include "rocksdb/options.h"
-
-namespace rocksdb {
-
-class LDBTool {
- public:
-  void Run(int argc, char** argv, Options = Options());
-};
-
-} // namespace rocksdb
-
-#endif // STORAGE_ROCKSDB_INCLUDE_LDB_TOOL_H