Fix multilistener issue (as reported in issue #34)

Attempts to fix the rerun premature exit reported in issue #34.

The main issue is caused by new API changes in the Listen gem.  Specifically, the Listen gem now requires start! to be called if blocking mode is desired.

Note with this change, I had to modify watcher_rspec.rb to do the path replacement from /var to /private/var before the Watcher init call.  For some reason, the listen gem doesn't properly ignore the default ignored directories (.git, .svn, etc) if a symlink is specified.

Note: I've only tested this on MacOS...