Add maxComplexity for mccabe.

This PR allows users to set the max-complexity argument to flake8 enabling the McCabe complexity checking flake8 offers.

![screen shot 2014-09-05 at 11 20 16 am](https://cloud.githubusercontent.com/assets/877908/4168071/990e8ea0-3518-11e4-8ed5-1795e2be271e.png)
![screen shot 2014-09-05 at 11 20 36 am](https://cloud.githubusercontent.com/assets/877908/4168072/9aba5f04-3518-11e4-90a5-86cce40ae1f7.png)