The `self.typingAttributes` will crash app in iOS6, it should be a bug of iOS6. Since typingAttributes only available in editing mode, checking editing mode previously will fix the bug.
New SAMTextView make the left padding nicely in iOS7, and the left padding of iOS6 is abandoned. I just move the left padding snippet of SSTextView here for the iOS6.
Merge pull request #11 from alvinhu/master

The `self.typingAttributes` will crash app in iOS6