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.

If we check by running it, why are we checking it before running it?

I think you can use the "lookupScopedKey" method on SbtClient for this, though it's possible it only gets tasks and settings but not commands. Assuming it works, I haven't exported it to JS I don't think, but it's simple to do. If it doesn't work on commands we might have to go add a way to look up commands to sbt-remote-control.