wip-149: Reading TTL and write time of regular columns

- Created case classes for different kinds of columns
- Changed the way the columns can be selected - select method now accepts SelectionColumn instead of String arguments
- Updated JavaAPI for TTL and write time retrieval support
- Updated and added test cases for TTL and write time retrieval support
- Updated API docs
- Applied reviewer's comments - merged ColumnSelection hierarchy into ColumnRef
- Add extractor to NamedColumnRef and use it.

Fixes #149
Merge pull request #468 from datastax/wip-149

#149 - Support for TTL and write time retrieval