Support the '--watch' compiler flag.
Addressed code review feedback.
Changed watchFile callback behavior.

We no longer recompile unless the new last-modified time is more recent.
Changed messages, got rid of 'oldProgram'.
Minor edits, no longer release 'program'

Edited comments, no longer releasing 'program' as it is potentially confusing and doesn't buy us a heck of a lot anyway.
Merge pull request #324 from Microsoft/watcherIardlyKnowEr

Support the '--watch' compiler flag.