Show a customer.

This is very basic, similar to what Rails generates by default. I like
using a table rather than <p>s, though.

Fixes #20.
Link to customer edit from customer show.

I like to test that these links are here. The test is simple, and this 'feature'
is important. Links are part of your app's usability.
Shops can only see their own customers.

Lots of duplication going on here. I'm leaving it all in for now, once
I have artists with permissions as well, we'll build a 'real' permissions
system.
Merge pull request #30 from artisan-tattoo/customers

Show customers