Product find by permalink

Replaced queries in the form of `Product.where(:permalink => id).first`with the cleaner syntax of `Product.find_by_permalink(id)`.

The motivation behind this was using Spree together with Globalize3. Globalize3 hooks into the dynamic finder methods but not into the `where()` queries. (I use Globalize3 for translating the permalinks.)

Apart from that, at least the first commit 85e9644 introduces stylistic improvements using the cleaner `find_by_x` syntax (not sure about the second commit 98b7b07) 

This should be fine to be merged into 1-2-stable as well.