Add new "register to volunteer" permission. Update access arguments for volunteer sign-up form to use this instead of "access civicrm."
Update volunteer sign up form to enable display of profile to anonymous users.
Resolves "Not enough data to create activity object," API error when anonymous users try to register to volunteer.
Added help text re the new "register to volunteer" permission.
Merge pull request #77 from fjg-conflux/anonymous_users

Make volunteer sign-up form accessible to anonymous users.