provide "modifier" (userid) index

[buchi]

use collective.lastmodifier

[gyst]

Irrespective of in which upstream the other indexes are maintained, I think this one should be PLIPed for inclusion in the Plone API.

[gyst]

This probably needs to be merged with #3 in order to prevent any inconsistencies between the userid and Date indexes on "real" modifications.

Given a document authored by user "foo" 10 minutes ago, when user "foo" adds a plone.app.discussion event, the userid index should return "foo" and the date index should return "10 minutes ago". Maybe rename the indexes ActualModifier and ActuallyModified or something like that.