Implement GIS Implicit flow for web

[axi]

An implementation with implicit flow which supports scopes.
There are still some things to figure out:

• Not sure about the refresh() case: is it still needed if grantOfflineAccess is not supported anymore ?
• As stated here, I didn't choose Authorization code flow because it would now require a backend platform to host an authorization code endpoint.

[axi]

This should fix #202 but I need some opinions on this one, @reslear maybe ?

[reslear]

Hey, cool, but we need more code flow also

[axi]

I don't get why we should have code flow, it wasn't the case before, is it ?

[sjdrew]

@axi thanks for this. Can we get this merged before google shuts us down?

[sjdrew]

@axi one minor suggestion. the scriptId you have as "gapi" but this is actually loading the GIS Library which loads itself onto the window as 'google'.

The GAPI Library, loads itself onto the window object as 'gapi' .

So would just be less confusing if you named that scriptId tag "gis".

[axi]

I don't know what to do next without guidance