Remove Poppler as a hard dependency

[bosd]

At the time this project was created there might be not a lot of options to extract the texts from an pdf. Poppler was/is a very good library for extracting the texts. But a common complaint/constraint is that it cannot be installed with a simple pip install. (and it's license) Now there are some more options for extracting libraries.

I'd like to propose to remove poppler as a HARD dependency. Instead use PyPDF as the default input module.