Use button i18n in test.
Use FactoryGirl to create test objects rather than doing so directly.

This avoids problems with attr_accessible being introduced.
Add message text to group membership requests.
Allow users to specify a message when asking to join.
Show joining message in notifications.
Show membership request messages on the review page and indicate they exist on the overview.
Use factorygirl to avoid mass assignment errors.
Merge branch 'membership-req-messages' into staging

Fixes #270