Add a stub policy that authenticates against a fixed password

To use the stub policy, set "openstax_accounts.stub = true" in
development.ini

The default username is "test" and the default password is "password",
but can be overridden by setting "openstax_accounts.stub.username" and
"openstax_accounts.stub.password".  The profile can be set in
"openstax_accounts.stub.profile" in json format.
Add support for travis