Add a link to create a customer on the dashboard

Pretty basic, and the same thing that we did for Artists.
New customers can now be created

Huge forms are hellish. Glad to knock the first version of this out.

I am considering factoring that date select into a *gasp* helper.

This all mirrors the customer creation stuff, there's just way more
fields for a customer.
Update a customer

You'll notice that I've moved the form into a partial, for re-use. Aweosme.

I don't like interpolating the ID in the CSS, but what are you gonna do.

I've also decided to keep the customer CRUD tests together, so I r-named
the test file to reflect that.
Merge pull request #16 from artisan-tattoo/new_customer

Create a new customer