Tesseract support for various image types

In reference to #14, #15, #16

Originally I thought about just checking for known image types in the process() method and routing it to one tesseract parsing module (as it's the same command) because I'm repeating myself a lot here but I didn't want to muddy up the code. 

Edit: Actually jpeg/jpg at the very least should be treated as the same, @deanmalmgren let me know