add BigInt column

BigInt column.
Inherits from Integer column.

Addresses this issue
https://github.com/cqlengine/cqlengine/issues/81

Doc changes now cite Integer as "32-bit signed integer" and BigInt as "64-bit signed long" per CQL docs
http://www.datastax.com/documentation/cql/3.0/webhelp/index.html#cql/cql_reference/cql_data_types_c.html