One client class (GraceDb) will now be usable for all authentication
types (X509, basic, and None).  Users can provide credentials to the
constructor directly (path to cert and key or proxy file, or
username and password). If those are not provided, it will look for
X509 credentials in environment variables and then in an expected
default location.  Basic auth credentials will be looked up in the
default location for a .netrc file.

The client instantiation will no longer fail if credentials are not
found (unless the user explicitly sets the 'fail_if_noauth' property
to True).