'Add explicit type' quick assist.

Add a quick-assist for adding an explicit type for `val`s and `def`s, if there isn't one already.

* it supports multiple assignment, but not pattern matches (not sure how useful it would be)
* tests depend on the UI (like all other quick fix tests). I think it's possible to decouple them in the same way we did  for code completions, but that's for another PR
* I added a few more general-purpose methods for getting back ASTs from positions, and even tokens. I'll clean them up for M2, but I think they can benefit the scalatest plugin as well

![screen shot 2013-10-13 at 8 12 42 pm](https://f.cloud.github.com/assets/133742/1322400/fdb06ecc-342a-11e3-8dce-c96f4718d59f.png)