Separated mplayer Function for Google TTS

This breaks the actual system call for mplayer off into its own function
called playGoogleTTS_mplayer. Breaking this off will allow calling this
code from a future separate class for threading purposes.
Move Google TTS Download to Thread Class

Takes the current downloader logic and moves it to a "worker" class
which is accessed by a "downloader" class. The latter is in charge of
firing up the thread and making sure that the user does not do anything
that would cause another thread for the same file to be started.
Thread Clean-Up

Adds a check to see if old threads are hanging around when fetch() is
called again, and if so, deletes them so that their destructors can be
called.
Merge feature/google-nonblock

Brings in the new threaded Google TTS downloads.