Update dependencies and drop scala 2.9 support in preparation for scala 2.11 support

This is some (untested) grunt work that I did in my unsucessful attempt at upgrading to Scala 2.11. I hope it's of use to get some of the nasty stuff out of the way. Here's what I did so far and why:

* Scalatest needs to be upgraded in order to support 2.11
* in turn, scalatest is not compatible with 2.9 anymore, forcing users to drop scala 2.9
* 2.11 also requires updating to twitter-server to 1.9.x, which changed the `Admin` trait to require `AdminHttpServer` too. There was an incompatible `AdminHttpServer` in this project that seemed unused, so I just passed in the upstream one and deleted the one from this project.
* Mustache.java [moved scala specific things to a subproject](https://github.com/spullara/mustache.java/pull/100), but still does not support scala 2.11
* They also seem to have removed the `TwitterObjectHandler` class, so I dropped it in here for now

Upstream twitter-server dropped mustache in twitter/twitter-server@0c64d37f2805d9ebdc49ea9ec0f46d2714e87991, not sure if this project wants to do the same or wait for them to support 2.11.