(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 handler for serving static content from the
base-path. To turn the created context to anything more advanced
one has to supply one or more objects implementing the
ServletContextInitializer interface which can then customize the context
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 starting
the newly created context handlers from all the context handler registration
functions into a separate function - start-context-handler - to prevent
code duplication.