Implemented TomcatRolesPrincipalProvider

Submitting this as a PR to start the conversation.  Not sure if this is enough or whether more is needed.

The use case as I understand it... we want to be able to configure roles in the Tomcat tomcat-users.xml file that will be treated as principals for access to resources.  A sample tomcat-users.xml file might look like:

    <tomcat-users>
      <!-- Fedora roles -->
      <role rolename="fedoraAdmin"/>
      <role rolename="fedoraUser"/>
    
      <!-- AIC roles -->
      <role rolename="aic-scraper"/>
    
      <user username="fcindexer" password="thepasswd" roles="fedoraUser"/>
    </tomcat-users>

With this TomcatRolesPrincipalProvider class, access to a FF4 object can be given to the aic-scraper role (as a principal):

    {
      "aic-scraper" : [ "reader" ]
    }

And the fcindexer will not be able to access the object.  When the aic-scraper role is added to the fcindexer user, though:

    <user username="fcindexer" password="thepasswd" roles="fedoraUser,aic-scraper"/>

the fcindexer will then be able to access that object.

This is tested with Tomcat7... if my understanding of the needs of this ticket is correct, I can check to see what versions of Tomcat (6, etc.) are handled by it, or whether more code is needed to support them.

I'm also not quite sure how to write tests for this since it's a container specific thing (i.e., checks a Catalina class that won't be found in our Jetty dev environment).  As you can see in the code, reflection is used to avoid container specific dependencies.

Awaiting feedback for possible revisions... thanks.