skip frames for phase shift

I add a argument `skip_frame` to sine_wave function for create phase-shift wave, that's a must-have feature to create same frequency but different phase waves in 2 channels.

See also:
http://rtfms.com/episode-4-turn-your-iphoneandroid-mac-pc-player-etc-into-a-universal-remote.htm
https://gist.github.com/xyb/5733797