add description for tox tasks

[gaborbernat]

For example:

tox -av 
default environments:
py27     -> run test suite for the application with python2.7
py36     -> run test suite for the application with python3.6
lint     -> run linter (prospector) to ensure the source code corresponds to our coding standards
docs     -> build readthedocs documentation

additional environments:
coverage -> run test suite with code coverage for the application with /usr/bin/python
eslint   -> run the JavaScript linter (requires gulp installed)

[ericholscher]

Thanks!