Add mousetrap.js file from Mousetrap 1.4.5, under the Apache license.

The file was download and verified via these commands:
$ git clone https://github.com/ccampbell/mousetrap.git
$ cd mousetrap; git checkout 1.4.5

The file in that repository named mousetrap.js is exactly the same one that
appeared in RhodeCode 2.2.5 in changeset c8d3c0d61d95.

The mousetrap.js states clearly that it is licensed under Apache-2.0.