Support consistency in Metal::DataSet

Consistency can be added as a chained scope to a data set; at that point, all
queries generated by that data set use the given consistency. Default
consistency can be set per keyspace.
Allow batches to have specified consistency
Accept consistency arguments for reads and writes at the Record level
Fix some deprecation warnings
Add consistency info to README
RBX fix
Merge pull request #89 from outoftime/consistency

Support for consistency tuning