new mic integration

Added WITMicViewController and supporting files (reachability class, waveform drawing class, WITResponse model object, sounds, and images). Users just need to create a child controller that is of type WITMicViewController and become it's delegate.

Other changes I made to SDK:
- Added code that allows the microphone to cancel the request when no speech is detected.
- Switched timer implementation from CADisplayLink to NSTimer so the mic can detect audio levels when app is in the background
- Modified the AVAudioSession to switch audio routes depending on whether headset was plugged in or not.