#149 - Support for TTL and write time retrieval

I thought of this a little more and I think we're duplicating concepts here. We are using the above hierarchy to *reference* Cassandra columns, either regular columns or virtual columns like TTL/writetime. We already have one class hierarchy to reference columns, which is used in `ColumnMapper` - the hierarchy rooted at `ColumnRef`. Why not unify these hierarchies and have just one `ColumnRef` for referencing cassandra columns like this:

```scala
trait ColumnRef
trait NamedColumnRef { def columnName: String } extends ColumnRef
case class ColumnName(columnName: String) extends NamedColumnRef   // former NamedColumnRef and PlainSelectionColumn unified
case class TTL(columnName: String) extends NamedColumnRef                 
case class WriteTime(columnName: String) extends NamedColumnRef 
case class ColumnIndex(columnIndex: Int) extends ColumnRef                       // former IndexedColumnRef, renamed in order not to confuse it with "index columns" which are something unrelated
```

This way we could support fetching TTLs and write times in the object mapper.