Merge branch 'release/0.17' into develop
add a django secret key so tests can be run with django 1.5
update version to 0.18.0-dev since 0.17 is current released version
add xquery support for filtering on node exists, exclude
xquery filters for >, < comparison
correct next release version in changelog
set version to 0.18.0 final in preparation for release
Merge branch 'release/0.18'