Presentation compiler no longer loads all project sources by default

This change isn't large but puts us 90% of the way towards fixing #362. It needs an ensime-emacs change as well.  @fommil, it would be great if you could try it out. I didn't try to apply this change to 2.9, let me know if it needs fixes.

BTW I had to fix the classpath problem mentioned in #610, it just won't work without it.

A new RPC call, :unload-all, removes all sources from the presentation compiler, then makes the PC reload the target's .class files. This is basically the opposite of :typecheck-all.

A separate mechanism is triggered when .class files are modified: it invalidates the classpath then reloads source files that the PC knows about. This has the effect of updating the PC's state when the project is recompiled.