Rewrite internals

* Fold request into client
* Use MultiXml in place of custom parsing logic (#17)
* Parse flat files (Closes #11)
* Optionally, stream response (Closes #12)
Rewrite API sections

* Flatten layout, remove requests and parsers (Closes #17)
* Remove VCR fixtures, simplify integration tests
Use yardoc
Update metadata
Add stubs for Cart and Customer Information
Update README
Bump version to 0.7.0