Raise NotImplementedError on unavailable functionality

Closes #17
Allow input_cred to be None in add_cred_impersonate_name()

Closes #19
Add gss_add_cred() including docs and test

Closes #15.
Support 'store' argument in Credentials.add

Previously, there was not way to access `add_cred_from` from
the high-level API.  Now, `Credentials.add` accepts the
`store` method.

Closes #14