This is a simple getter and setter of the OscillatorType enum, with
error checking to not allow 'custom', as that should only be changed
through 'setPeriodicWave()'.
This is still missing a bunch of spec steps to construct the
audio node based on the parameters of the OscillatorNode, but it is at
least enough to construct an object to be able to add a basic test which
can get built upon as more is implemented.
