Do not query database for non-spatial tables

This PR avoids unnecessary SQL queries of `geometry_columns` for tables that do not have any spatial columns.

Fix #71

* Add a private SpatialColumnInfo class to manage the spatial column
info query and memoization.
* Add mocha to test expectations.
Could these two lines (38, 39) become the following?

```
all[column_name]
```

What are we gaining from the memoization?  Is `spatial_column_info` used elsewhere?