The spec isn't _super_ clear on how this is meant to be done, but the
way I understand this is that we should simply clamp the returned
'current value' between 'min' and 'max'.
Firefox does not appear to do this clamping, but Chrome does.
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.
