Using Session objects, refresh, and misc stuff

Moved to using session objects instead of directly calling request methods and letting the library handle it, because requests-oauth purely uses sessions.

This appears to be completely incompatible with vcrpy, so now the tests don't work.

Also added a refresh method to explicitly re-retrieve subobjects (e.g. some_product.refresh('images') ). Not a nice solution, but it's either this, refresh every time, or try to figure out a way to refresh on every change (for which I can't see a non-overly-complex solution for).