modified class so it could be overriden with user speicifc changes

When I was using nano to create a custom webserver, some of the methods in SimpleWebServer prevented me from being able to customize the implmentation for my own needs. Since SimpleWebServer is not an "example", it should be modified to allow the user to custom it for their needs.

"registerPluginForMimeType" was originally private. It prevented me from adding custom plugins without changing the base class directly. I also needed to be able to add those plugins when the server was instantiated. Since, I used a anonymous class, I added the "init" method to help accomplish that.
The get responses were added so error messages could be customized based on the actual design of the server. Same reason for changing "listDirectory" to protected.

This request should be thought of as start to changing this class slightly to better allow customization. I only modified the parts I needed, so there may be more areas where this could be improved.