preparing a first release under the name `c_geohash` (version 1.1.2)

* tests now use mini-test (for Ruby 2.x support)
* C extension can be compiled and tests run using rake tasks
Merge pull request #2 from mapzen/preparing-a-c_geohash-release

preparing a first release under the name `c_geohash` (version 1.1.2)