Add http event support for Google Cloud Functions

[pmuens]

Right now we only support the (event, callback) function signature for Google Cloud Functions.

However Google support the (request, response) function signature for http events. The emulator should support this type of function as well.

Since request and response are Express objects (see Google Docs here) we can simply add the express npm package as a dependency and update the Google Middleware to pass the request and reponse objects as the function parameters.

The middleware-part which needs to be updated can be found here (an if-else which checks for the event type can be implemented so that the functionParams for the http functions are computed as well).

/cc @ac360 @mthenw