Fix bugs in invitation policy module (python3 compat.) and GETMETADATA

Summary:
Workaround bug in GETMETADATA response parser (no literal strings support)
by requesting only folder-type annotations when we need only these. It should
also improve performance.

Fixed various python 3 compatibility issues.

Fix

Reviewers: PyKolab Developers, mollekopf

Reviewed By: PyKolab Developers, mollekopf

Subscribers: PyKolab Developers

Differential Revision: https://git.kolab.org/D4451