Feature: Add taxons to prototypes

This PR is a new feature inside of Spree: The ability to add taxons to prototypes.

This was done by creating a HABTM association from the taxons to the prototypes. Then, I edited the admin interface to be able to include the creation of the taxons within the prototypes. When a new product is created, it will now import the taxons from the prototype's taxons.

While I was creating the taxons/prototypes association, I was also using the properties model for an example. There was a method in there, `find_all_by_prototypes` that was broken. I have removed it completely as I was not able to get it working TDD wise.

The prototypes feature spec contained CRU, but no D. I added a D to give it the full CRUD.

The method for creating the associations within the prototypes for products was called `add_properties_and_option_types_from_prototype`. Since it now does taxons, it needed some renaming. It is now called `add_associations_from_prototype`. Much nicer.

`Spree::Property` and `Spree::Prototype` did _not_ have any model specs. I also needed to be able to test the `has_and_belongs_to_many` associations, so I hooked up shoulda and made this possible. It should allow the application to increase in the number of (mostly redundant, but definitely effective) tests for validations and associations.

I also noticed the `to_param` method inside of the `Spree::Taxon` model that had a comment that said `# For #2759`. I took the liberty to make a test for this method since it did require one. It was a bug fix and shouldn't be removed. I was originally going to use `alias_method`, but it appears it would call onto `self` (or rather `Spree::Taxon`) which was kinda weird.

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If you're visual, here's the before:

![screen shot 2014-10-12 at 5 28 30 am](https://cloud.githubusercontent.com/assets/3117356/4606042/6aab9fca-520b-11e4-85f8-86217154e519.png)

And the after:

![screen shot 2014-10-12 at 5 29 07 am](https://cloud.githubusercontent.com/assets/3117356/4606044/73539498-520b-11e4-8404-06a90ed59399.png)