Add some unit tests

I wanted to do some larger refactorings, but felt uncomfortable without some existing test coverage so I tried to add some ChefSpec tests around this cookbook—the coverage is by no means complete but I'm hoping it's a good start. You should be able to invoke the tests with `rake test:unit`.

You might have to do something like [this](https://github.com/opscode/chef-dk/issues/106#issuecomment-50245127) if you have issues with Faraday and SSL.

I'm hoping to do more work refactoring. However, I figured those changes might be objectionable and I think the value of these tests hopefully stand by themselves.

I moved a bunch of methods from default provider into the helper module to make substituting test doubles easier, hope that's okay.