Stub implementation of smarthome intents

[marcuson]

See https://developers.google.com/codelabs/smarthome-washer/#0 for a template project to follow (this one uses Firebase)

[marcuson]

@labodj some minor issues:

• functions/src/smarthome/i-smarthome-devices.ts: usually there is only one interface definition per file, where the filename is the lower kebab-case version of the Pascal case class/interface name.
• functions/src/smarthome/smarthome-devices.ts: there could be no need for interface implementation, we'll talk about this later when we'll introduce GCloud Datastore.
• functions/src/smarthome/i-smarthome-intent-responses.ts: here we can use a simple interface with only properties (no methods) and create a service which will implement a method to return such response (instead of using functions/src/smarthome/smarthome-intent-responses.ts).
• functions/src/smarthome/google-smarthome.func.ts: the service of the previous point should be injected in the constructor of the controller (do not use new keyword).

also some Typescript best style practices:

• interfaces usually do not start with I prefix (maybe I've used this format in some example code, my bad), so e.g. ISmartHomeDeviceGeneric should be SmartHomeDeviceGeneric