Properly handle partial update of resource attributes in PATCH requests

[jathanism]

For Resource objects (Device, Network, Interface) that are able to have attributes, attributes need to be properly handled.

We should be able to specify a dict of only attributes that we wish to change and have the API properly reconcile them on a PATCH request.

For PUT requests, everything is working as intended. For reference internally, a PATCH request is a an update with the partial=True flag set, where PUT is an update with partial=False.

[jathanism]

The correct solution involves moving all of the attribute "patching" from the pynsot.app.App.process_attributes() method into the NSoT backend.

This way, the API can do the right thing with regard to PATCH requests, and pynsot can become even dumber, which is a good thing.

[jathanism]

PATCH support was added to API in #155. Now we can talk about partial updates of attributes, if that's still a desirable feature.