Update aadhaar.py and pan.py + Add Indian EPIC and GSTIN

[vairag22]

• aadhaar.py Improved validation and docstrings, and did other changes as per PEP8
• pan.py Improved validation and docstrings, and did other changes as per PEP8
• epic.py Added support for handling EPIC number (Indian voter identification number)
• gstin.py Added support for handling GSTIN (Indian VAT number)

[vairag22]

Took 3 bad pull requests (#276, #277, #278) but finally I'm done. It was a good learning experience though.

[arthurdejong]

Hi @vairag22

Thanks for your contributions.

I've merged your changes with some modifications. Specifically I reverted some of your changes to Aadhaar and PAN to maintain the current API (e.g. renamed card_holder_type to holder_type but also kept the old name to not break third-party code that relies on that).

In the GSTIN module I delegated some of the checking to the PAN module to avoid duplication.

Btw, you can easily run the tests locally with tox which should save you some time in pushing commits to test them.