Do not query database for non-spatial tables

Fix #71

* Add a private SpatialColumnInfo class to manage the spatial column
info query and memoization.
* Add mocha to test expectations.
Move SpatialColumnInfo to a separate file

* It’s now a proper class. Private classes are not a thing.
* Remove unnecessary ‘.rb’ from requires
Test spatial column info caching
Remove unnecessary attr_accessors
Merge pull request #125 from rgeo/spatial_column_info

Do not query database for non-spatial tables