* Add necessary dependencies, build script, and `NSSpeechSynthesizer` backend.
* Get very basic speech working.
Needs a delegate to handle queued speech, and currently segfaults if one is set.
Two Tolk instances were being created. One checked for the presence of a screen reader. The other actually performed the speech, and was returned as part of the `TTS` instance.
Unfortunately, Tolk doesn't seem to appreciate being called twice. So here we check if a screen reader is detected and, if one is, return the instance that did the detection. Otherwise, error out and return the WinRT backend.
I'd previously attempted to normalize everything to `u8`, but this had some drawbacks:
* It failed to account for some synthesis drivers defining normal as mid-range, while most define it very low.
* It didn't track the normal value for a given synthesizer.
* There was no clean way to map a curve between the minimum, normal, and maximum rates.
Here we track the minimum, normal, and maximum values of rate, pitch, and volume. Sanity checks are done on set.
Also, as a further proof-of-concept, all properties are now implemented for the WinRT driver.
