Organized code

[yichizhang]

Adding accessibilityHint when initializing with title and message. (previous version only adds accessibility information in "initWithMessage:") Organized methods in implementation file, used pragma mark to put methods into different groups. There is a bit of repeating when calculating the bounding size of message and title. Put them into two separate methods. Changed a spelling mistake. ( 'timeInvertal' -> 'timeInterval' ) Changed 'colourHL' variable to 'colorHL'. (should use American spelling when coding)