Added and improved documentation.
Refactored InvalidColumnCount and its use.
Added TableData::Table#fetch_cell, which replaces #cell.
Added TableData::Table#fetch_row, removed TableData::Table#cell (replaced by #fetch_cell).
Removed TableData::Table#to_nested_array, changed #to_a, updated docs to reflect changes.
Changed TableData::Table#== to only consider contents, added #eql? and #hash which consider content and metadata.
Code cosmetics.
Updated TableData::Column#[] to only consider body values.
Updated TableData::Column#each to return self.
Added TableData::Column#each_value.
Changed TableData::Column#== to only consider contents and work with Array and to_ary objects, added #eql? and #hash which consider content and class.
Added a dev comment.