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
Display OIDs in dotted form

This commit causes `repr` for OIDs to display the OID in dotted
form instead of as the raw ASN.1 BER-encoded bytes.

Also-Authored-By: Simo Sorce <simo@redhat.com>
Support constructing OIDs from integers

This commit introduces support for constructing OIDs using integers
(instead of having to pass the BER-encoded bytes to the constructor).