Fixed issue #21

Sorry about this.  I realized my use case was ONLY if I had a SZTextView in a .xib.  If you have a SZTextView setup in a .xib and have a contentInset set, it needs to be set up right when initialized.  

Here is the issue I was having for reference. 
![ios simulator screen shot oct 13 2014 10 54 37 am](https://cloud.githubusercontent.com/assets/1844728/4618095/56b5f9a6-5306-11e4-82ef-7a33990ffa23.png)