Wassim

I did this to avoid copying the tests and make sure that the new argument `parallel` does not break the API.

I did implement a multi-threading layer above camelot.read_pdf using **multiprocessing** library. I faced a couple of pitfalls doing it, so I can help on this if I may.

Hi all. Sorry it took me a while to publish the PR while the code was already available. Now the PR is there to be reviewed, I'm looking forward for...