Add drawer methods to recenter view on a specific position (or percentage).
Don't create AudioContext before init is called.

If AudioContext and webkitAudioContext aren't available, simply including the
webaudio.js file will causes errors. Instead, we don't attempt to create the
context object until the webaudio object's init method has been called.
Pause playback but don't reset position when end of audio is reached.

When the playback reaches the end, pause playback and set the paused position
to the duration. If #playPause is called again, detect that we're already at
the end and restart playback from the beginning.
Keep audio paused (if currently paused) when seeking.
Add method to stop (and reset) playback.
Add method to test if the object is loaded/ready.
Add method to clear markers and add an update method to markers.
Merge remote-tracking branch 'katspaugh/master'

Conflicts:
	src/drawer.js
Merge pull request #17 from jabr/master

Assorted features and fixes