Added customizable action sheets to the photo browser

It adds this optional method to the delegate

- (void) photoBrowser:(MWPhotoBrowser *)photoBrowser actionIndex:(NSUInteger)index;

Set the actions this way:

browser.displayActionButton = YES;
browser.actionButtons = [NSArray arrayWithObjects:@"Save", "Copy", "Flag", nil];

Thanks for the great library. This gives us the ability to customize the action sheets and implement whatever kind of things we want. I've updated the readme as well. Only bad thing is that the default "Copy" etc are not ever called.