Restart presentation compiler instead of invalidating classpath

It would be great if @fommil could try this change on his large work project, to make sure the concept works before I finish this PR.

What's working (at least in scala 2.11):
- restart PC when you type `C-c C-c r` or when compilation happens
- saner handling of continuous compilation (thanks for the hint @fommil)
Performance is great when working on ENSIME itself

What's missing before it can be merged:
- thorough testing
- rewrite type-id caching  (right now swank:type-by-id won't work across restarts)
- check for any other leaks of Type, Tree or Symbol objects