Subscribe to Google Fit Activity

[ibraude]

I noticed that I must have Google Fit on my device to record activity.

I wanted to have the module record activity even if the device does not have Google Fit installed, so I added a subscribe function that writes user activity to the Google Fit Store.

It should be initialized when loading the app via Fitness.subscribe(), it returns a promise to show the status of the subscription (true for success and false for failure).

[scerelli]

@ibraude it seems nice to me, can you also update the docs with the new methods? so we can test and merge this.

Thanks

[ibraude]

@scerelli Will do.