Prerelease of IdP 3.1.0 available

[Tom Zeller]

A prerelease of IdP 3.1.0 is available[1].

The idea is that interested parties can test to their satisfaction
over the next day or two and we can vote to release on the Friday dev
call.

Note that version numbers are '3.1.0' and do not include 'snapshot' or
'prerelease' which may be a little confusing, but pending any issues
the prerelease will simply become the official release.

Obviously this is not the documented or usual release process, but in
order to vote-to-release I think we need the thing-being-voted-on to
be available.

[1] http://shibboleth.net/downloads/prerelease/