Add basic support for GCP auth

[johnpoth]

This adds basic support for kubebox to authenticate via Google Cloud Platform (GCP) #50
I've changed the way authentication was implemented. Now providers are called and can modify outgoing HTTP request through the provideAuth method. This should make it easier to add other providers in the future.

This PR still has some limitations:

• Scopes aren't supported
• Browser not supported (command has to be run locally).
• Time format isn't taken into account and relies solely on Javascript's Date parser.

Thanks!

[johnpoth]

Thanks for reviewing. PR updated!

[johnpoth]

Updated the PR, thanks!

[astefanutti]

Thanks!