Allow mp3 sounds to be added to database

This was previously only loading in the .wav filenames.  Now
that mp3 files can be played, we also want to load in the mp3 filenames
Properly extract the extension of filename

In order for the sound to know whether it is loading a wav file
or an mp3, it needs to know the extension.  I'm pulling it off when the
song is initialized and passing it to the loadSound function.
Use the libMAD library to decode mp3s

These changes allow mp3 sounds to be loaded and decoded into the expected
wave form.  There's an ugly global tempBuffer, which could probably be
refactored and improved.  I mostly just didn't want to give each sound its
own tempBuffer.
Add libMAD library

This is the mp3 decoder library as provided by the TAs.  These files
shouldn't be changed.
Remove pointless printf