Support cross-building with Scala 2.11

Since Scala 2.10/2.11 are source compatible, we should be able to cross build for both versions. From what I understand there are basically three things we need to figure out:

1. Have a two versions of our dependency graph, one that uses 2.11 dependencies and the other that uses 2.10 dependencies.
2. Figure out how to publish different poms for 2.10 and 2.11.

I think (1) can be accomplished by having a scala 2.11 profile. (2) isn't really well supported by Maven since published pom's aren't generated dynamically. But we can probably script around it to make it work. I've done some initial sanity checks with a simple build here:

https://github.com/pwendell/scala-maven-crossbuild