Added grails.cached.resources.excludes

Hey there, I partially implemented one of your TODOs for this plugin by adding an "excludes" array that takes Ant-style regular expressions.

For example:

```
// Don't rename and apply special cache headers to PNG files
grails.cached.resources.excludes = [
    "*.png"
]
```

I didn't do the dynamic closure part yet so I left that in the TODO list.

Haven't done much Grails plugin development so let me know if I did something wrong, otherwise hope this will be useful. I needed it because we're doing Retina support via a front-end JS (and the grails-retina plugin wasn't an option for various reasons).