Bug 865256 - Part 2: Rename WaveTable to PeriodicWave; r=roc

--HG--
rename : content/media/webaudio/WaveTable.cpp => content/media/webaudio/PeriodicWave.cpp
rename : content/media/webaudio/WaveTable.h => content/media/webaudio/PeriodicWave.h
rename : content/media/webaudio/test/test_waveTable.html => content/media/webaudio/test/test_periodicWave.html
rename : dom/webidl/WaveTable.webidl => dom/webidl/PeriodicWave.webidl