Sku enhancements

[MatteoPiovanelli-Laser]

This implements the first stage of enhancements discussed in #101. Pasting from there:

sets up a feature that has:

• A setting to enforce checks on the uniqueness of SKUs.
• A setting enabling automatic generation of SKUs:
  • The generation works similarly to Autoroute, but it is based on a site setting, rather than a type setting.
  • The default pattern is "{SKU-{Content.Id}"

With the difference that the default pattern used is SKU-{Content.Slug}

The settings for this go in a menu under Commerce -> Settings

[bleroy]

Thanks for the changes. I'll let you decide if you want to address the new comments. In either case, I'll merge.

[bleroy]

Thanks!