Split the server-specific code from the module code

@wheresrhys This feels like an ok pattern for the server side stuff.

One oddity is that you can't use [utils.inherit](http://nodejs.org/docs/latest/api/util.html#util_util_inherits_constructor_superconstructor) because that will overwrite all the functions added to the prototype, so I had to [copy the methods over instead](https://github.com/Financial-Times/ft-poller/compare/master...poller-isomorphic#diff-e6a5b42b2f7a26c840607370aed5301aR8). Seems to work ok.