License strapless under MIT, like Bootstrap.

This changes the README file to the new “Copyright and License” statement [from twbs/bootstrap](https://github.com/twbs/bootstrap/blob/master/README.md#copyright-and-license).

Use Grunt to automatically keep the `package.json` license information in synch with [that of twbs/bootstrap](https://github.com/twbs/bootstrap/blob/master/package.json).