Detect and receive notification of more iOS accessibility features

Adds following methods for detecting iOS accessibility features:
MobileAccessibility.isGuidedAccessEnabled()
MobileAccessibility.isInvertColorsEnabled()
MobileAccessibility.isGuidedAccessEnabled()
MobileAccessibility.isInvertColorsEnabled()
MobileAccessibility.isMonoAudioEnabled()

Adds corresponding window events to subscribe to notifications of
accessibility feature status updates:
“guidedaccessstatusdidchange”,
"invertcolorsstatusdidchange”,
"monoaudiostatusdidchange"

Adds following method for posting a notification as a string to be
announced by VoiceOver:
MobileAccessibility.postAnnouncementNotification()
Merge pull request #2 from majornista/master

Detect and receive notification of more iOS accessibility features