Add Pandas extra to our setup.py

[Zac-HD]

Justin H reports on Stack Overflow that pip install hypothesis[pandas] fails - and he's right! Fortunately this is an easy fix; all we need to do is add 'pandas': ['pandas>=0.19'], as an entry in our setuptools extras dict. It would also be good to add a minimum version to the pytz requirement for the Django extra!

If this is your first contribution to Hypothesis, don't forget to add your name to the list of authors in CONTRIBUTING.rst and to write a RELEASE.rst file for a patch release! You can read the ./guides in the repo, or just copy a recent pull request 😄