DTActionSheet: Moved block execution to clickedButtonAtIndex:

Moved block execution from 
    - (void)actionSheet:(UIActionSheet *)actionSheet didDismissWithButtonIndex:(NSInteger)buttonIndex
to
    - (void)actionSheet:(UIActionSheet *)actionSheet clickedButtonAtIndex:(NSInteger)buttonIndex
and added corresponding delegate checking and bitmask updating code.

Avoids a very noticeable delay in executing the requested action.
Merge pull request #63 from obviousengineering/dtactionsheet

DTActionSheet: Moved block execution to clickedButtonAtIndex:
Updated Change Log for late addition to 7.1.
Merge branch 'release-1.7.1'