Added getNumberofTracks() to MP3Trigger-for-Arduino library

Hi Carl,

Thanks for your work on the library. It has been very useful in getting the MP3 Trigger up and running.

I added function using Status Request command to get the number of tracks on the SD
card. It seems multiple users may benefit from this function and the basic code for receiving serial communication from the MP3 Trigger. e.g. [MakerJam Forums] [1]

Thanks for including.

 [1]: http://makerjam.com/forum/mp3-trigger/arduino-serial-communication/