Added flexibility to printFile

I made a minor tweak to this addon so it's easier to target files to print, whether they're in your data/ directory or not.

I also added in some installation instructions. The forum post wasn't clear whether you needed to configure and make CUPs or gutenprint. If one were to try to configure and make CUPs and the build failed bad things could happen to their system. Hopefully the README will help clarify what people need to install and what they don't.

Out of curiosity is it possible to just send raw data to CUPS? Similarly is it possible to specify other sorts of file formats, like docs or rtfs? While it's pretty reasonable to assume anyone using openFrameworks + ofxCUPS is probably printing something visual in nature it'd be nice to create flexibility for other kinds of file formats.