Make mysql an optional dependency

[willprice]

To enable pattern to be installed on a diverse range of computer systems where optional dependencies' dependencies like mysql_config aren't present we should make those dependencies optional, e.g. mysqlclient.

People can then install the full set of dependencies by pip install pattern3[mysql].

Let me know if other dependencies are optional and I can add them to the setup.py script

[willprice]

Closed in favour of #250