Various Small Fixes

This PR covers:

- `Sensor` and `Actuator` now have a `name` field (human-readable name) (to be added front-end, @mcerrillo)
- `Sensor`'s `measureType` is now a string (as requested by @dbuldon)
- `GET api/sensors` and `GET api/actuators` now accept new optional parameters, `type` and/or `customId`, to return corresponding devices (as requested by @dbuldon)
- `POST api/simpleRule` allows to create a simple Rule binding only 1 sensor and 1 actuator (check `services.js` to see the request's parameters) (as requested by @mcerrillo)
- Already-added devices will be initiated (by calling their drivers `add` method) when the server (re)starts (as suggested by @bim9262)