Methods to render the PDF within a certain size preserving aspect ratio

[mac-cain13]

Created fitSize: methods that will render the PDF image fitting within the given CGSize, but always respecting the aspect ratio.

What did we do?

• Added imageWithPDFNamed:fitSize:atPage: and imageWithPDFURL:fitSize:atPage: methods (and the variations without atPage:)
• Updated Readme.md so it mentions the fitSize: method

I think these methods can be useful for a lot of people, so I'll hope you'll accept this pull request! Let me know if there is something we should update before you can accept the pull.req.!

[mindbrix]

Many thanks for this. Great addition.