MRI compatibility for Pdf417

I'm trying to make this gem as cross platform as possible so I decided to switch to Hoe to create the gems since it's not really stable yet and I don't want people using it directly.

In this PR I implemented Pdf417 in MRI using the C lib.

Just FYI, next up on the todo list is to be able to generate DataMatrix barcodes in Java either by removing the semacode dependency or porting semacode in Java.