change name of at_least_one_parameter decorator to at_least_one_of
Merge branch 'master' of github.com:plone/plone.api
add at_least_one_of to content.py and update tests.
Correct exception message when at_least_one_of is violated
add at_least_one_of to group.py and update tests.
add at_least_one_of to user API
Merge branch 'master' of github.com:plone/plone.api