Refactor base class to be service-agnostic

Eventually, the gem should ask the user which services to integrate. For now, the base class `TimeLogRobot` makes calls to `Toggl::Report` and `JIRA::WorkLogger`, which is not great. The calls instead should be made to non-namespaced services, which then inspect some param/argument and route to the proper service accordingly. See the `Entry` and `Tagger` models for examples.
