#8 invitations

[Jochen Staerk]

this patch (against 1.6.1) should contain my last 2
submissions (show-results-on-survey-completion and
hidden-multicopy-functionality) and a new, bigger
feature that I call invitations.

With invitations, you can invite a group to a survey.
The members of the group will receive an e-mail with
the URL to access the survey and their credentials. An
important feature is that phpESP then keeps track of
incoming responses, i.e. one can see on the status page
of the respective invitation when, and more importantly
if users have completed the survey.

A word of warning: Realms seems to be shallow in the
1.6.1 phpESP architecture (has that changed to 1.7? I
think there is a login-form now?). It seems as if both
user and realm are part of the primary key in the users
table, but due to http constraints, it can not be
determined which group a user belongs to when he/she
loggs in, so de facto the user name is the only primary
key. One might run into trouble if there are users in
different groups that share the same user name.

If user "A" group "A" is invited to Survey "A" and
another Username "A" in group "B" is also invited to
Survey "A", their participations would be recorded as
the identical user in both group A and B.

[Jochen Staerk]