oauth2: move Token into its own file (no code changes)

Change-Id: I5248f48cc1af68a0f78a8ddf7e28b91a729f5460
Reviewed-on: https://go-review.googlesource.com/1870
Reviewed-by: David Symonds <dsymonds@golang.org>
oauth2: use a JSON struct types instead of empty interface maps

Change-Id: Ifd66ea35c15dbd14acca0c945b533ec755de12e4
Reviewed-on: https://go-review.googlesource.com/1872
Reviewed-by: Burcu Dogan <jbd@google.com>