GEOS-5607 fixed exception throwed by nested layer group in Container mode
doc: updated geoserver-manager link
documentation: layer group updates
Merge pull request #123 from svzdvd/GEOS5607-groupcontainerbreakpreview

GEOS-5607 fixed exception throwed by nested layer group in Container mode
Merge pull request #124 from svzdvd/geos5548-documentation

Documentation
Merge branch 'master' of git://github.com/geoserver/geoserver