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.
I would prefer to keep compat to minor version, not releases. But I must say that `Widget::Checkbox` is very welcome.