DefaultParticipationSetter: do not require browser layer.

This makes testing and customization easier. We are specific enough.
Refactor invitation accepting tests.
Do not set default Reader role when single roles is configured.

When the option "allow_multiple_roles" is disabled, the "Reader"
role, which is always added by default, should not be added.
Merge pull request #27 from 4teamwork/jone-reader-optional

Do not set reader role when not using multiple role option