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.

my point here was that "undefined" isn't a good way to refer to "unset" because it looks like a bug (some JS engines used to warn about referencing an undefined var even). setInterval never returns 0 (spec says "a user-agent-defined integer that is greater than zero") so you could do `maxProcessTime !== 0` after initializing to 0, or you could use null. undefined can happen due to "spelled the variable name wrong" but null/0 cannot.

clearInterval/clearTimeout do silently no-op if given an invalid value, but I think it's clearer to check for validity ourselves, personally.