Update `TableReader` to handle `schema_columns` returned from Cassandra 2.0

Cassandra 2.0's interface is actually a lot nicer, since it returns *all*
logical columns (including keys), instead of just data columns. However, for
now it's cleanest to just ignore non-data columns from that query, and just
keep the existing approach to introspecting key column structure, since that
works in both 1.2 and 2.0.
Documentation for `Metal::Logger`
Style points
Merge pull request #60 from outoftime/2-0-4-compatibility

Compatibility with Cassandra 2.0.4