Eliminate a race condition where a second alert is displayed after the first is dismissed but before dismissBlock is run

This race condition led to the second alert disappears shortly after it is shown, since the dismissBlock
from the first alert hides the alerts’ window by calling [self.previousWindow makeKeyAndVisible]
Fixes a potential crash that can occur when hiding the keyboard as a result of dismissing an alert
Add an extra level of indirection in order to support iOS 6 by using UIAlertView natively
Implement addSubview on SDCAlertViewWrapper so that consumers can add views to the contentView easily
Fix a problem with how delegates were passed through to UIAlertView and SDCAlertView.

We make sure that the SDCAlertViewWrapper’s delegate is passed an instance of SDCAlertViewWrapper;
before this change we were passing the actual alert view.

Includes some fixes to make sure that the instance of SDCAlertViewWrapper is retained by something
during the time that its alert view is visible, since that should be the expectation of consumers of this class.
Ensure that on iOS 7, the alert never grows larger than the screen.
Remove unnecessary logging
Set a named constant for vertical margin
Make sure that status bar doesn’t change style when showing alert
Merge branch 'status-bar-style' into thr-status-bar-style

Conflicts:
	SDCAlertView/SDCAlertViewController.m
Merge pull request #1 from sadlerjw/thr-status-bar-style

Add iOS 6 support via a wrapper