Introduce Tax Code to Tax Category

This PR introduces a simple Tax Code field that can be used to manage important information that maybe meaningful to something external to Spree, i.e. Avalara, IRS, Accounting Team, etc. The current fields of name and description don't really cater towards keeping "code" like information around.

http://developer.avalara.com/api-docs/designing-your-integration/gettax

In my particular case, I need to associate a code to a shipping method for Avalara, i.e FR0000000 in order for me to properly calculate shipping taxes (in another project) and I'm not sure I want to keep it under name or description, nor do I want to decorate the shipping_method model directly.

I know this could just belong as a Spree decoration in my application, but seeing if there's an interest to make this a part of core.

What's contained in this PR.
1) Migration.
2) Simple controller tests for tax category.
3) Changes to index and _form for tax category.