Feature/set_operations: Added Thrift operator support. 

- Fixed bug in OptionalColumns. Bumped version to 0.2.0
- Improved greedy initialisation of objects in ```CassandraTable```
- Improved operator support for Thrift Columns.
- Added an ```OptionalThriftColumn``` and a ```ThriftMapColumn```
- Added more unit tests for query serialisation and operators.
- Bumped version to ```0.2.0``` and published to Maven Central.