Add basic support for GetFeatureInfo

I've tried to use existing code as much as I could. I've made a huge refactoring work in GetMapHandler because most of the parameters that are mandatory for GetMap are mandatory too for GetFeatureInfo.

One huge change is what I've done with parameters. For GetMap and getFeatureInfo, I've created a dedicated class that takes the Map of HTTP parameters and that creates a suitable Java object from it. Thanks to that, I can use inheritance for this parsing operation and I can simplify some methods signatures that were, well... complicated.

I've added some tests, too, where I could...

Methods in AbstractGetHandler come from the factorisation work. They were part of a really huge method in GetMapHandler that I have split for the sake of simplicity.