+ Added LevelDBStore to store persistent data.
 + Added LevelDBStoreIterator, the iterator on LevelDBStore
 + Added test cases for LevelDBStore.
 * Fixed autoconf scripts and travis.yml for correctly building process.
Merge branch 'master' into leveldb_store

Conflicts:
	.travis.yml
 * Prettier formating
 * Use `const auto& ` in the range-loop
 * install boost 1.53 from ppa
Merge branch 'master' into leveldb_store

Conflicts:
	.travis.yml
 * Fixed the PPA for boost 1.53
 * Update README.md for the new dependancies and versions
 * install boost-filesystem and system as well
Merge pull request #3 from vsfs/leveldb_store

Add LevelDBStore to store the data in a on-disk K-V Store.