Slatwall v3 integration

I've added the Slatwall v3 integration into your URL Tools plugin. If enabled, this would allow for alternative URL's per product in Slatwall when installed into mura.

Additonally I've added a plugin setting which decides whether the URLTools are responsible for adding the canonical to the head or not.

![a60d277a-f59e-11e2-82e3-66c3bd4e58a8](https://f.cloud.github.com/assets/392542/921123/749d81ae-feee-11e2-98e3-3701b0b4c4b4.png)