AttributeFilter: support Regex in attribute values

Hi guys,

One more pull request from me: I've had a go at adding support for regexp in eduPersonEntitlement values to support Groups.

Was quite easy to do - and I've done a corresponding test too.

One gotcha this has: I could only do this for the attribute filter, NOT for the metadata.  Unfortunately, there is no way for SAML 2.0 metadata to specify regexp values - there is only the generic type AttributeValue - which in this case would contain a regexp instead of a value.  Any thoughts on this?

And any comments on the patch itself?

Cheers,
Vlad