Support for full-screen mode on Mac OS X

Implemented support for monitoring Hearthstone in both windowed *and*
full-screen mode on OS X. Improved window detection means that
monitoring should continue no matter how many times you change
resolution or switch between windowed and full-screen mode.

Note that this change only works for OS X — it has no impact on Windows
which still supports windowed mode only.
Merge pull request #179 from gtch/master

Implemented support for full-screen mode on Mac OS X