#149 - Support for TTL and write time retrieval

I like this idea. Actually I was thinking about transforming this stuff even more - why not encapsulate mapped field name in `ColumnRef`? We could be able to make something like this: 
```scala
sc.cassandraTable("ks", "tab").select("id" as "key", "value" as "data", "value".ttl as "dataTTL")
...
```
This would be very convenient, much more than it is now. 
wdyt?