Raise a LoginError if expected cookie not set from SE OpenID login.

Raise an error if an HTTP error is returned in _post_with_fkey().
Adds live tests for SE OpenID login, ensuring success and failure are recognized.

This test is only enabled if login credentials are found in the
environment, which they should be on manishearth's Travis config.
Merge upstream/master into master.
Merge pull request #12 from jeremybanks/master

Raise an error for invalid SE OpenID logins (includes live tests)