Fix Issue #29 - Handle http_auth of None

talons.auth.basicauth.Identifier.identify() was not properly checking
for cases when falcon.request.Request.auth was None and not the expected
tuple of (auth_type, user_and_key). We patch up the identifier to handle
None and add a unit test to prevent regressions.
Merge pull request #14 from jaypipes/issue29

Fix Issue #29 - Handle http_auth of None