Better minification

These modifications attempt to achieve better minification, while fixing some bugs:

- use UglifyJS for the minification to properly expose the `CM` object and its methods
- cache the date object used in `unset` function
- cache JSON and JSON.stringify
- use `this` instead of `CM`
- change deprecated `escape` and `unescape` functions with `encodeURI` and `decodeURI`
- fix incorrect `expires` parameter name in `setObject`

The minified result is bigger than the current file (1.126 kb instead of 1.068 kb) but the closure compiler that I think you used broke the script.