Relicense jquery.placeholder.js to MIT

Relicense jquery.placeholder.js to MIT

I would like to relicense this code to make it possible to merge it
into https://github.com/mathiasbynens/jquery-placeholder , which is
another placeholder library implementing some features we're missing
and missing some features we have.

Older versions of this file will naturally still be licenseable under
the GPL v2 license.

The following people touched this file, and I have asked them all for
approval. I have also traced the ancestry before the file appears in
this repository and removed authors of reverted patches.

* Trevor Parscal
* Timo Tijhof
* Roan Kattouw
* Marius Hoch
* rillke

Bug: 47057
Change-Id: Ic04291cecb20cc03d48565cf5586da849eb8e962