Fixed some jnotify loading issues

Fixes #2620.

This change does two things:

* Ensures that jnotify is loaded lazily.  This means on platforms where
  loading jnotify throws an exception, the exception is only thrown when
  you go to run Play, it's not thrown immediately when Play loads.
* Ensures that after an SBT reload, jnotify still works.  It does this
  by making the classloader that jnotify is loaded in (which is a hack
  that dynamically adds a new library to the library path) available
  globally through the platform mbean server.