Make spree_tax_cloud play nice with other tax rates.

This stops spree_tax_cloud committing tax cloud transactions for orders that do not use the tax cloud calculator. Stores that ship into and outside the US are likely to have multiple tax rates, with only the US tax rate being applicable for orders into the US.

This may not be the cleanest solution but it does fix the problem I'm having using spree_tax_cloud in a store that has multiple tax rates.

To get the test passing I had to add the states to the US zone. I think spree was defaulting to the US zone previously. With multiple zones it needs to be given a little more information to pick the right zone.

Unfortunately I haven't been able to verify the other tests. I can't get to taxcloud on the network I'm on at the moment :confounded:. I'd appreciate it if someone were able to verify the tests are still ok. 
Quoth TaxCloud CEO David Campbell: "[TaxCloud] should ignore product/item properties that allow a merchant to click a check-box to make something taxable or exempt."

My intention, following that directive, was to make this gem as independent as possible from Spree's default `TaxCategory` specifications, and the reason is that it's just not clear how they apply cleanly in a setting as complicated as the US. What's tax-exempt in one state might not be in another, and so forth.

As the README explains, Spree::TaxCloud is designed to function in a single TaxCategory, and it is expected that _all_ Products and all ShippingMethods will be in the _same_ TaxCategory as the one configured for the TaxRate using the Tax Cloud calculator above (in this example, `Taxable`).

You can see via grep that TaxCategory is not used anywhere other than the fact that by necessity the TaxRate needs to make reference to one, so I've suggested that users just create a single TaxCategory in which _all_ their Products will go. This extension will not work as described when there are multiple TaxCategories.

One solution would be to give TaxCategories each a TaxCloud TIC that applies to those Products in that TaxCategory... I decided that this implementation was too complex and potentially inexact, and decided to scope the TaxCloud TICs instead to a single global default for items, which can be overridden at the _Product_ level (rather than the Category level). I can imagine certain stores preferring to use Categories rather than Products, but the arguments can go in either direction, and at some point you just have to commit to an implementation and let people override if they wish, so I went with Product-scoping.

I'd actually _love_ to revise the implementation to lift the single-Category restriction, as I'm assuming that European sellers in particular (maybe including yourself) need to use TaxCategories in their European Zones.  So, for instance, I'd love to make it so that the TaxRate with the TaxCloud calculator is overridden such that Spree thinks it applies to _all_ TaxCategories. 

If you have any ideas in that vein, I'd be really excited to make some steps in that direction.