Scala 2.11 compatibility.

- Fix Shape.copy type
- Remove illegal val in ColumnExtensionMethods
- Convert \0 to \u0000
- Change TupleClass handling in SlickBackend
- Move ShapeLevel._ types to the top level to avoid diverging implicits

Tested with 2.10.0-M6 and 2.10.0-M7.

Review by @cvogt, /cc @retronym