Use ShamRack to stub responses

* Removes XMLSimple
* Removes FakeWeb
* Removes VCR
* Adds ShamRack
* Adds Sinatra (as test dependency)
Refactors `Solusvm::Base` to use JSON API
Refactors `Solusvm::Node` to use JSON API
fixup! Test response structure
fixup
Refactors `Solusvm::Server` to use JSON API