Support for Scala 2.10 and 2.11

As has been discussed on the mailing list I have been working on making adding support for Scala 2.11 and also allowing working on both Scala 2.10 and 2.11 projects with a single Ensime distribution. This pull request contains all of those changes.

The new Ensime distribution (created with "+ stage" in sbt) now looks like this:

dist
     /elisp
     /2.10
            /bin
            /lib
     /2.11
            /bin
            /lib

When ensime mode is entered an ensime server of the proper version is launched based on a new setting in the configuration file (:-scala-version). The ensime-sbt-cmd project has already been updated to create the :scala-version tag based on sbt project files.

The only known issue with this release is that build/rebuild project is no longer hooked up to the build reporting system. This is because the BuildManager, which IncrementalBuilder depended on, was deprecated in 2.10 and removed in 2.11. Unfortunately, the intended replacement, Zinc, did not support everything Ensime needed (e.g. access to the reporter) and is only supported in 2.10 projects. As Zinc and SBT are updated we should be able to make very good use of them.