Convenience function to create a client

Loads a config file at and creates a client according to various options. Handy IMO so you don't have to constantly look up your user key etc. Has some precedent in a similar system used by boto for AWS credentials (http://boto.readthedocs.org/en/latest/boto_config_tut.html)
Clarified some of the terms
init() in get_client()

I think it makes sense to init() in here too, and allow the user to keep the api_tokens in the config file.
Addressed changes
Merge pull request #5 from sbirch/patch-1

Convenience function to create a client