Allow disabling and enabling IR emitter from application code.

This allows application users to enable and disable the emitter. This is useful when in a multiple-sensor environment, where you want to synchronize the IR emitters to avoid interference. Starting and stopping the depth stream can take up to a second, whereas using setEmitterState takes .005-.25 seconds. 

The one questionable thing this contains is making SetEmitterState public rather than private, but other than that, I tried to follow the same style as used by setting the mirroring state.