Fixed bug where pitch wheel events were being masked incorrectly

For example, if the variable pitch passed to PitchWheelEvent.set_pitch was 0x7FFF then data[0] would be set to 0x7FFF & 0xFF which is 0xFF.  This results in an event which has a data byte which is not 7-bit, which, for example, results in an invalid MIDI file when you write it out.
Merge pull request #17 from craffel/master

Fixed bug where pitch wheel events were being masked incorrectly