Add listen gem for filesystem change polling.

I am still working through some acceptance test failures at the moment, but I wanted to get it out for some feedback (mostly to make sure I am moving in the desired direction).  

This specifically should address issues #75 and #67.

A couple of specific notes:

* I added `listen` to the gemspec as a runtime dependency.
* I changed ApplicationWatcher#globs to #directories since we are only using it in that sense.  The listen gem requires the root directories themselves to be set when starting listening, but we could still get a glob style matching by using File.fnmatch? if we really want that functionality.
* I added `rb-inotify` and `rb-fsevent` to the Gemfile for both Spring and the test application.  These can be removed, but it made testing easier to have them there.