Added cross-compilation for 2.10 / 2.11 Scala version

Added cross compilation capability to build and made changes required for it to build on both Scala runtimes. Also changed default Scala version used to be 2.11. 

If you run:
`sbt compile test publish` - it will run with 2.11.0

If you want to do cross-build for all versions (2.11.0 & 2.10.0) you prepend tasks with `+`:
`sbt +compile +test +publish`

This also actually solves the problem https://github.com/anvie/blueprints-scala/issues/23

There is one question left:
* how to make it available through maven central ?