Added auth module with function for retrieving the user's CADC id from
their certificate file.
Refactored ProgressManagers to store the userid as a field, which can 
optionally be passed to the constructor.
Get the CADC username from the issuer common name by stripping 
keycodes.
Switched progress managers to use CADC username instead of Unix 
username.
Merge pull request #147 from drusk/cadc_id

Closes #142.