Doc improvements: update version numbers, provide example build.sbt

Hi.  Based on some hiccups I had getting started with twitter-server, I thought I'd make it easier for others to get started.
* updated version numbers to reflect recent releases.
* rather than update versions in docs automatically (I'm new to all of this), included link to CHANGES, so versions should be more obvious to new users.
* included a minimal build.sbt example, using current version numbers.
this is not quite right--let's drop the _2.10.