relicense to MIT

[BurntSushi]

Closes #183

[ryanwalls]

@BurntSushi Just curious. Why COPYING instead of LICENSE? Our automated tools for license checking don't pick up the COPYING file... so I have to manually come here. Then I couldn't find the license file, so I search issues to find the issue about switching to MIT #183. Then I had to look at this PR that closed that to then find that the file was called COPYING. Whew. :)

[BurntSushi]

@ryanwalls COPYING is a standard name for the license file. (Even Github picks up on it and correctly labels this project as MIT.)

[ryanwalls]

@BurntSushi Ah. Good to know. Didn't realize it was standard. Will send a PR to the licensing tool I'm using. https://github.com/pivotal/LicenseFinder

[ryanwalls]

@BurntSushi Since this has come up before, see kubernetes/kubernetes#31024, it would seem a small change to standardize. But I understand if it is a low priority.

Thanks for the great tool!