Use pipenv for managing developement virtualenv

[ob-stripe]

r? @brandur-stripe
cc @stripe/api-libraries

This PR ended up being larger than I intended, so here's some helpful information:

• pipenv is a fancy new tool for managing development virtualenvs. It's fairly recent, but is officially recommended by python.org. (It's also developed by Kenneth Reitz of requests fame, and is used by requests itself.)
• Pipfile and Pipfile.lock work similarly to Gemfile in Ruby projects. I've committed the lock file because it's recommended (cf. git - Should Pipfile.lock be committed to version control? pypa/pipenv#598) and is what's done in requests.
• Added a Makefile for common operations
• Updated .travis.yml to use the new make targets
• Updated tox.ini to set up a separate environment for flake8 linting

[stripe-ci]

Gerald Rule: tell fred about apple pay changes

cc @fred-stripe

[ob-stripe]

This line basically means that pipenv will run python setup.py install, which will pull the install dependencies from setup.py according to the Python environment.

[brandur-stripe]

Any idea what "e1839a8" is?

[ob-stripe]

It's a short hash of the path (in this case .) automatically generated by pipenv to avoid collisions if you have multiple entries. I think it can actually be anything unique -- I'll try renaming it to stripe for clarity.

[brandur-stripe]

Do you mind if we remove the init target in favor of just having this duplicated in .travis.yml and the README.md? I feel like many Python people will already have have a pipenv available, and there are other recommended ways of installing it (e.g., I used Homebrew as recommended on their page).

[ob-stripe]

👍

[brandur-stripe]

Very cool! I just tried it to make sure it wasn't too much trouble, and your instructions in the README.md went off without a hitch :)

I left a few comments above, but otherwise LGTM.

[ob-stripe]

Okay, I made the requested changed. Going to pull this in.