Refactor Trello Driver to match standards

[nicoespeon]

I guess my current implementation of the Trello Driver is not standard, according to the docs:

The output of the driver function can either be a single Observable or a queryable collection of Observables. […]

In some cases it is necessary to output a queryable collection of Observables, instead of a single one. A queryable collection of Observables is essentially a JavaScript object with a function used to choose a particular Observable based on a parameter, e.g. get(param).

− http://cycle.js.org/drivers.html

Let's think on that and refactor: advices and suggestions are welcome!

I might also extract the Driver into its own repo if it happen to be relevant.