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)
Semantically, raising IOError would be more explicit:
`raise IOError(2, "No such file", config_path)`