HST Spring Security Plugin - support authentication bypass in Channel Manager

As discussed in [1] in the Hippo Community group, it would be good to implement an authentication bypass in the Channel Manager.

Now, since this can be achieved by setting 'hst:channelmanagersiteauthenticationskipped' to 'true' for repository-controlled authentication, it would be a logical step to extend this support to the HST Spring Security Plugin.

This for example allows a different/pluggable authentication framework to be used on the site, while enabling editors to maintain the site using the Channel Manager.

[1] - https://groups.google.com/forum/#!topic/hippo-community/01f8C1c_OvI