Renamed Main.storyboard to KPGallery.storyboard

This will make it less confusing when adding KPGallery to another
project.
Images are now referenced by NSString paths in the dataArray instead of kept as UIImages

The dataArray will now hold NSString path references to images instead
of storing UIImages themselves. This is significantly more memory
efficient.

In the process, more methods were added which will help with
abstraction, resulting in less duplicate code.
Added ability to save images

You can now automatically save images to a desired directory or to the
cache. This is mostly useful for when wanting to store and keep a
remote image instead of only using it temporarily.