Precalculate a map of column names -> column indices

This allows us to avoid using Array#index to map names to indices, which
requires N**2 comparisons between column names to fully hydrate a row.
Performance improvements are considerable for large result rows.
Remove #column_index helper; use #column_indices to check for int name
Fix column_family_spec to not check for column.id attribute now absent in cassandra 1.2
Merge pull request #43 from outoftime/column-name-map

Precalculate a map of column names -> column indices