Version 1.7

So I've looked at all the open issues and tried to fixed them all. I think this does it.

I've fixed compatibility problems with Symphony 2.5 (#17) and Implemented uniqueness (#7).

#14 can be close IMO.

This PR creates version 1.7.0, **which is 2.5.x compatible only**

Please review! But I might merge real soon as I have a couple of projects needing this.
You can remove this block altogether if you wish. We don't have it for other Symphony extensions anymore and are just using the `extension.meta.xml` file for this information.