The ability to choose the completion method

Sometimes developers have to use other `UIAlertViewDelegate` methods, such as `-alertView:didDismissWithButtonIndex:` for better user experience.

The ability to choose the completion method in `_RMAlertViewDelegate` would be a great solution.

I added an enum (`RMErrorRecoveryPresentedErrorCompletionMethod`), which has three values:

- <code>RMErrorRecoveryPresentedErrorCompletionMethod<strong>ClickedButton</strong></code>
- <code>RMErrorRecoveryPresentedErrorCompletionMethod<strong>WillDismiss</strong></code>
- <code>RMErrorRecoveryPresentedErrorCompletionMethod<strong>DidDismiss</strong></code>

I also added a new method in `@interface UIResponder (RMErrorRecovery)`:

    - (void)rm_presentError:(NSError *)error completionHandler:(void (^)(BOOL recovered))completionHandler completionMethod:(RMErrorRecoveryPresentedErrorCompletionMethod)competionMethod;

The default value of `completionMethod` is still `RMErrorReco...ClickedButton`, so it is compatible with the old version.

I updated the `README.md` as well, so it contains the information about completion method.

I hope you'll find my pull request useful. :)