Add calculator availability

This branch adds an `#available?` method to the base activeshipping calculator. It also renamed `#compute` to `#compute_package` allowing the base class' `#compute` method to coerce the object to a package.

This fixes a bug where if the API doesn't return shipping rates for a given calculator (which I've confirmed happens in USPS' api) the shipping method is skipped over. Without this, I would see an error message like this:

![screen shot 2013-09-11 at 11 52 22 am](https://f.cloud.github.com/assets/133/1127666/d011fe6e-1b42-11e3-8b22-b98ab1c84f0c.png)

I will be submitting a separate PR to spree to resolve the display issue.