standardized text extraction tests

@PrajitR mentioned something like this a while back and I think it could be really useful (https://github.com/deanmalmgren/textract/pull/30#issuecomment-51436069), particularly in light of issues like #48 where we were inadvertently "extracting" text that wasn't actually in the original file. It might be nice to have a set of tests that should have the exact same output, regardless of the filetype, so we can be confident that each extraction method works consistently.