SHA-256 and fetching https documents

[Mark Doliner]

FYI I've written an implementation of SHA-256 and I've made
modifications to our purple_util_fetch_url_request() function to be
able to fetch https documents.  If either of these things would be
useful to people I can try to check them in soon--just let me know.
If not I'll probably just wait until later (they're needed for the
newer clientLogin AIM authentication process, which I still think we
should try to use).

The SHA-256 implementation is clean.  I based the code on our SHA-1
implementation and modified it based on the documentation published by
NIST.

And I don't remember if the purple_util_fetch_url_request() function
breaks our API.  I think it doesn't.

-Mark