Different host for some APIs

[gwendall]

Some APIs (like Contacts for instance - https://developers.google.com/google-apps/contacts/v3/reference#Feeds) use a different _host than googleapis.com. We should be able to set it through options.

[danopia]

I'm in need of this, doesn't seem like an uncommon usecase - #7 apparently was struggling with it

Included in danopia:google-api for those in need

[merlinstardust]

This is also needed for Classroom.

A workaround is to add a line similar to this in your app
GoogleApi._host = 'https://classroom.googleapis.com';