Update contentInset properly, set a specific size for the pull to refresh view

Hi @samvermette,
Here's a pull to request to update the value of the contentInset when it's modified after the pull to refresh view has been created.
(Alternative implementation would be to add a method to update originalInsetTop and originalInsetBottom).
Also, I hardcoded a new value for the size of the pull to refresh view. Of course, that's probably not the right thing to do. Do you recommend a way to move forward to be able to customize its size?
My goal when changing the size was to move the arrow outside of the screen since I changed the contentInset value of my table view.