oauth2/google: use the metadata package, cleanups
oauth2/google: use the metadata package, cleanups

Verified it compiles on Go 1.2 now too.

Fixes golang/oauth2#70

Change-Id: I099a86676d2464b3840f1798bbca914a202eb195
Reviewed-on: https://go-review.googlesource.com/2372
Reviewed-by: Burcu Dogan <347c19a639ce752a783b4ea325d6539ab4b4a262@google.com>

paul-at-start/oauth2,golang/oauth2,walmartlabs/oauth2,freeformz/oauth2,danielkarch/oauth2,cw2018/oauth2,jgalilee/oauth2,helojo/oauth2,broady/oauth2,qtzheng/oauth2,Kuzmin/oauth2,EagerIO/oauth2,flowlo/oauth2,hoonmin/oauth2,lancecarlson/oauth2,dsriram/oauth2,zj8487/oauth2,tpiha/oauth2,pengwynn/oauth2-1,dbmedialab/golang-oauth2,equinux/oauth2,jboeuf/oauth2,cllu/oauth2,steveruckdashel/oauth2,djalal/oauth2,heshed/oauth2,wmydz1/oauth2,hoysoft/oauth2,awormuth/oauth2,drewwells/oauth2