Wip/h3nk3 shims

Moves the shim functionality from sbt-rc into Activator.

When generating project files there are multiple steps, some synchronous and some asynchronous. Therefore the implementation is sort of state machine (see generator.js). Lots of mutable state and other ugliness due to the fact that it is implemented in JS... The process still feels a bit brittle but I think it is good enough to start the discussions around.

Atm, the final step in the process does not work because of an issue with SbtConnector that I am investigating. Since the issue seems unrelated to this PR I want to go ahead with the PR anyhow to get feedback on what has been done. 

For review by @havocp and @Warry.

I think eventually sbt server should be smart enough to restart itself, but right now it isn't. However, activator 1.2.x will notice a new .sbt file and self-restart. I'm not sure whether this is working or broken in wip/sbt-server, but if it's broken we should probably fix it (in AppActor.scala I believe). So I think in this file you can just assume adding/changing a .sbt file will cause a restart... after a short delay, that is.