Implement Database#schema; use to handle Dataset#columns in simple cases

@colinmarc

The spec for `Database#schema` is [here](https://github.com/jeremyevans/sequel/blob/509827867ec53b50229022a0fd7248faad319556/lib/sequel/database/query.rb#L80) -- it calls the db-specific `#schema_parse_table`.  I wasn't sure how to determine the following values for impala:
* :allow_null
* :default
* :type (a translation of the db-specific types to ruby classes)

Also, this implementation of `#columns` is not what one is supposed to do but (a) the standard implementation doesn't seem to work and (b) it solves my problem for the moment.

Let me know if you're comfortable merging with the above deficiencies or if you'd like me to address them beforehand.

Thanks!