Use the unlicense instead of CC0 to represent the public domain

I'm not sure how keen you are to have this conversation, but I didn't even realize there were more than 3 licenses before! Ack!

When I helped create the Unlicense a few years back, it was based on the dedication of the most successful public domain software project, SQLite. I can imagine that with public domain still a minority, we don't want to clutter the field with varying implementations of it. But I would humbly suggest that the Unlicense is a better example of a public domain license:

 * Its dedication is based on SQLite's, one of the top 10 software projects of all time by any metric.
 * It's actually a public domain dedication instead of a license emulating one.
 * It's shorter and clearer.
 * It seems to have greater adoption than CC0 (probably because CC was dis-recommending the use of CC0 use as late as 2011):

[356 cc0 results](https://github.com/search?q=cc0+path%3A%2FLICENSE&ref=searchresults&type=Code)
[1080 unlicense results](https://github.com/search?q=UNLICENSE+path%3A%2FUNLICENSE&type=Code&s=indexed)

You may disagree, in which case, go ahead and close the pull. I just wanted to have the conversation :)

This pull would be the one to address github/choosealicense.com#36 if we wanted to.