Add support for *easier* multi-akka builds

Updates to sbt scripts to support multiple akka version builds.  Locally this seems to work *as expected*, but please kick the tires on it a bit and see if it works well for you too :)

I've also created a [gist](https://gist.github.com/scullxbones/9713171) which has a sample shell script to drive this from command line.  I didn't commit because i'm not sure how you drive things like releases, and wasn't sure you wanted this in the project.

Heavily leveraged @eed3si9n [answer](http://stackoverflow.com/a/18383353/939126) and [answer](http://stackoverflow.com/a/20909594/939126) at stackoverflow.  Thanks Eugene!