Add credentials import-export to low-level API

Previously, there was support for credentials import-export in the
high-level API, but no actuall corresponding support in the low-level
API.  This commit fixes that.

Closes #25

Also-Authored-By: Robbie Harwood (frozencemetery) <rharwood@redhat.com>
REAME Updates

This adds a list of team members to the REAME,
as well as a Travis CI build badge
Release buffer after export_name

The low-level `export_name` function was missing a call to
`gss_release_buffer`, which could have resulted in a memory leak.

This is now fixed.
Normalize 'creds' argument name

Previously, a couple methods used 'cred' instead of 'creds'
as argument names.  Now, all methods use 'creds' (including
'acceptor_cred' becoming 'acceptor_creds')