(PE-2650) add add-context-handler function

This commit adds the ```add-context-handler``` function which is used by ```trapperkeeper-ruby``` to register rack based ruby applications with the webserver service.
When the function is called with only the ```base-path``` and ```context-path``` parameters it creates a context handler capable of serving just the static content from the ```base-path``` location. To turn the created context to anything more advanced one has to supply one or more objects implementing the ```ServletContextListener``` interface which can then customize the context handler by setting its init parameters, and by adding servlets, filters and other listeners to it.
Additionally the commit factors out a piece of common code for registering newly created context handlers with the webserver-context from all the ```add-*-handler``` functions into a separate function - ```add-handler``` - to prevent code duplication.