Require absolute import in kerberos.py.  

In 2.7 relative imports are the default, so referencing the name "kerberos" in the code
turns into a mess.  See https://github.com/requests/requests-kerberos/issues/8.

This fix simply requires absolute imports.  This makes it so that
"import kerberos" will always refer to the kerberos package/shared
library, and not the requests_kerberos.kerberos module.

It doesn't seem to break anything else currently.