updated setup.py with option to install tests

[George-Ogden]

now installs pytest with

pip install pretty_midi[test]

to make development easier

was hoping to make other modifications but that might be a future project

[craffel]

Thanks, any reason you think 0.6.0 is needed?

[George-Ogden]

There isn't an important reason for this - most people will end up installing the most recent version of pytest. I include it because I have done on previous projects.