Fancy password protected feed detection/support (bug 225)

Fancy password protected feed detection/support (bug 225)

cache.py: Make the fetch function aware of 401 errors as well as a channel's
    username and password attributes. The function will create the
    appropriate url if it needs to.

gui.py: Add a new dialog to handle username and password input.
    Modify add_new_channel to be aware of 401 errors generated by a
    podcastChannel's load() function. This means that the user no longer
    needs to enter a funky-looking url to get password-protected feeds to
    work. If a feed requires authentication, the user will be prompted.

    Fix a bug where the "add podcast url" entry wasn't reset after adding
    a feed that looked like a web page.

libpodcasts.py: Make podcastChannel.load() throw a HTTPAuthError exception
    if a 401 error occurs when downloadind the feed.