add tox environments that have the user specify what tests should be run

[beckjake]

This adds some environments to tox where the user specifies what tests to run (one each for python 2.7/3.6).

This is mostly useful when you want to locally run some single test in the integration suite and not wait for everything else to finish.

[drewbanin]

nice @beckjake. I think you can remove the coverage flags, as these envs will mostly be useful in dev. I find it annoying to have to scroll past the coverage report to see the actual failures. Just my opinion though, and this LGTM