Created fitSize: methods that will render the PDF image as big as possible within the given size preserving the aspect ratio

- Added imageWithPDFNamed:fitSize:atPage: and imageWithPDFURL:fitSize: methods
- Updated Readme.md so it mentions the fitSize: method
Credits to github.com/rickpastoor for implementing this methods. :)
Merge pull request #8 from wrep/master

Methods to render the PDF within a certain size preserving aspect ratio
Fixed PDFView mediaRectForURL: calls

All calls now request the rect for the correct page.
Invalid resource path/url will return nil UIImages

before this commit the App just crashed on invalid paths.
Merge pull request #9 from wrep/master

Prevent invalid path from crashing the entire app and get correct mediarect