Allow TTFunk to open an IO object

This PR gives TTFunk the ability to open an IO object. The code used in [self. verify_and_open](https://github.com/prawnpdf/ttfunk/blob/io-object/lib/ttfunk.rb#L18-35) is copied directly from Prawn's [Prawn::Images::verify_and_open_image](https://github.com/prawnpdf/prawn/blob/master/lib/prawn/images.rb#L131-L148). This could indicate a need for a 'prawn-support' gem to keep these helper methods DRY across our projects. @sandal if you can comment on if that is something we want to tackle now or just run with the copied solution here.