Partial logout feature in upcoming IdP release

[Cantor, Scott]

As some have possibly observed, I have been checking in code for a logout
handler in the IdPv2 branch. This was originally going to be an extension
Chad was planning to do, and because of other decisions, we have reopened
some work on V2 for another feature release for the first quarter of next
year.

Logout is a contentious rathole, and I'm not going to revisit it, but I
wanted to call attention to it for a couple of reasons.

- more testing
- vetting of behavior

I have completed a write up of the feature here:
https://wiki.shibboleth.net/confluence/display/SHIB2/IdPEnableSLO

I have been very meticulous in describing what this does and does not do.
If something is unclear, please ask and I'll do my best.

If you'd like to test this, you will want a clean install of the branch,
and to do that, you'll need snapshot builds of all the dependencies, but
other than that it's not a big deal. Just checkout the branches, do a mvn
-DskipTests install, and then the IdP should build.

If you disagree with, or wish to discuss, any of my design choices, I'll
listen, but I will say up front that there's very little chance of much of
it changing other than in minor ways. I scoped this out very carefully and
it pretty much does what I intended it to do. If I got something clearly
wrong, then obviously that's different, so that's why I'm pointing this
out well ahead of anything shipping and so people can plan.

-- Scott