Use GOOGLE_API_CLIENT_ID and GOOGLE_API_CLIENT_SECRET

[ccmaymay]

Hi rakyll,

Thanks for writing the drive tool---it's very nice. :) I changed Init() to read GOOGLE_API_CLIENT_ID and GOOGLE_API_CLIENT_SECRET, respectively, from the environment. If either is empty, an error is returned. This is minimalistic, and won't work for everyone, but if you like it feel free to merge. (This is also my first foray into go so it may not be very idiomatic!)

Chandler