Allow Blank Password in Basic Auth

This change allows a blank password for basic auth.  The exists() function is just the code generated by coffeescript's existence operator.  Blank password is required for the particular API I am working with (recur.ly).

The username could be made to allow blank through the same mechanism, but I can't imagine there being an actual use case for it, so I left the username validation as it is.

Fixes #91