Bump versions of datacash and paypal extensions

This should let us build the demo site in Python 3.
Use Python 3 compatible MySQL database adapter

* Oracle's connector does not work with Django 1.7
* MySQLdb has no Python 3 support
* The Python 3 support fork of MySQLdb doesn't support Python 2

So, good fun. But more googling turned up PyMySQL, which works a treat.
No idea why that isn't documented given the current poor state of
database adapters for MySQL.
Disallow failing tests with Python 3 and Django 1.7

This marks the end of my work on Django 1.7 and Python 3. Support for
both can now be considered stable.
Clarify that Python 3 support only works with Django 1.7
Merge branch 'mh/demopy3'