Bug: addButtonWithTitle fails when 0 buttons passed in initWithTitle

If you use ```addButtonWithTitle``` to add additional buttons rather than pass them all through ```initWithTitle:message:delegate:cancelButtonTitle:otherButtonTitles``` then the indexing of the buttons is incorrect. Maybe ```addButtonWithTitle``` wasn't designed to be a public method used this way, but I've found it's helpful sometimes to iteratively add buttons.

eg.

```
TTAlertView *alert = [[TTAlertView alloc] initWithTitle:@"test" message:@"testing alert" delegate:nil cancelButtonTitle:@"cancel" otherButtonTitles:nil];

...

[alert addButtonWithTitle:@"button that should show"];
[alert show];

```