Add Google App Engine support. First, http.DefaultTransport is not
available on GAE so appengin/urlfetch is used. Second, when running on
GAE Prod, service account and private key are no longer required. Instead
GAE App Identity API is used.
Rename WithContext and add comment
Update package document and README.
Add support for verify email OOB code.
Bump version to 0.1.1
Fix transport_test.
WidgetURL now is not required.
Change ValidateToken to return a *Token.
Fix GenerateOOBCode.
Merge pull request #4 from mcduan/master

Add Google App Engine support and Fix GenerateOOBCode