Custom Group Attributes

Hello cschiewek,

This is a fleshed out version of the group_attributes branch by dpmcnevin that allows a custom group attribute to be specified in the ldap.yml file.  This option can be turned on by adding the line "config.ldap_custom_group_attribute = true" to the devise.rb file.  The generators and documentation have been updated to reflect the changes.  

Integration with an Active Directory backed LDAP service prompted these changes.  (My AD server's group attribute is "member" rather than "uniqueMember".)

I also commented out the incomplete test, "With default settings use group for authorization should not be validated if group with different attribute is removed."

Thanks,

Mattbot