Moved custom Guice Injector creation to GuiceResteasyBootstrapServletContextListener + documentation

I deleted the GuiceInjectorResteasyBootstrapServletContextListener that I had created in a recent pull request because at the time I hadn't seen that the getModules() had been made protected to allow for customisation. So I added a withInjector(Injector) method that allows you to interact with the Injector after it's been created.

Also, I added documentation about the new configuration options and about the JaxrsModule, which was undocumented.