release 0.4.6

* Import lmdb 0.9.14 source.
* Handle MDB_MAP_RESIZED during transaction.
* Expose mdb_env_set_mapsize() as Environment#mapsize=.
* Cleanup and bug detection in environment_free.
* Add accessors:
   * Transaction#env
   * Database#env
   * Cursor#database
* Add Cursor#next_range for iterating up to a specified key.