Added GUI portion of dialog for getting a new CADC certificate.
Merge branch 'master' into retries
Added function for downloading a new CADC certificate given the 
username and password.
Added VOSpaceErrorHandler class which is used by the download 
manager.  Using this the application can now gracefully handle 
CADC certificate problems.
Set focus on username field in certificate dialog so that the user doesn't 
have to click anywhere before entering their credentials.  Made the
get certificate button default so that it is triggered on pressing enter.
Added basic test case for VOSpaceErrorHandler.
Merge branch 'master' into retries
Merge pull request #87 from drusk/cert_error_handling

Closes #49.