Cross build for Scala 2.11

This adds support for Scala 2.11, cross building for both 2.10 and 2.11. Tests pass on both builds, although there are lots of deprecation warnings when compiling with 2.11. Many of them are straightforward renames, but the most significant is the deprecation of Context#enclosingTree and friends, which are slated for removal without replacement in 2.12. (See [Changes in Scala 2.11](http://docs.scala-lang.org/overviews/macros/changelog211.html) point #29 in the Macros guide.) That's going to fundamentally affect how these macros work. In the short term, though, this does fine.