dataTypes.timestamp string encoding

Per http://www.datastax.com/documentation/cql/3.1/cql/cql_reference/timestamp_type_r.html, allow users to pass in a string that can be parsed as a date to represent a timestamp. Added some tests as well.

One note: depending on the environment it may reject a format that CQL considers valid. However it will throw if it cannot parse the string so it should not produce any unexpected values.

Great library, thanks!