Add support for the Unfuddle hosting service.
This supports Unfuddle as a hosting service. Unfuddle provides Git and

Subversion hosting.
Unfuddle's pretty straightforward to support. It makes use of HTTP Basic

Auth, and requires base commit IDs for posting.
There's also some improvements to RepositoryForm to provide more data to

the HostingService's authorize and check_repository functions.

The Unfuddle support uses this to do better validation of a repository

and an account, which we should make use of for other services.

Tried adding an account with bad credentials. Saw a reasonable error.
Added a valid account with no problems.
Tried to create a repository with bad repo info. Saw a reasonable error.
Created with valid info and it validated and saved. Tested this with Git and Subversion.
Posted diffs for both Git and Subversion. Both worked.