add setup.py and versioneer

[tyarkoni]

@chrisfilo: this is a start toward #1; I've added versioneer and a setup.py file. Do you mind setting up PyPI release from CI? I can also do it if you prefer, but I figure you'd want to use your credentials, plus I have no experience fiddling with circle's config.

I'd recommend changing the common folder to bids_validator instead of bids_validator_common for clarity and brevity, but left that up to you to decide.

[nellh]

This looks good but I do agree with the rename to bids_validator vs bids_validator_common, @bGrass is working on merging this into the main repo along with that change.