Allow uri-template arguments to be specified with **kwargs.

The methods for retrieving resources using uri-templates will now check
keyword arguments before the 'values' kwarg.

Previously the replacement keys would be passed in to the 'values'
kwarg, e.g.:
    root.get_review_request(values={'review_request_id':1})
This patch allows for a much cleaner syntax, e.g.:
    root.get_review_request(review_request_id=1)

Additionally, a more descriptive exception is raised when a template
argument isn't specified.

Retrieved review_request, review, and user resources using kwargs to specify
ids. Called methods with missing arguments. Mixed specifying the arguments
with kwargs, and values={...}.