+ Put records into leveldb when create/delete file/dir
 + Added namespace::remove_subfile
 * update persisetn store when add/remove subfile
 * Fixed a typo
 * Initialization
 * Added iterator to TestLevelDBStore
 + Added test case for Namespace::init()
 * Fixed typo
 * Correct comments
Merge pull request #30 from vsfs/store_namespace

Save and Load namespace to/from leveldb