Prevents username and password from being exposed in error tracebacks.

This is only safe as long as the username and password are never stored
in any new variables, but are only passed directly as

    browser.loginSEOpenID(live_testing.username, live_testing.password)

This change includes a test change because that test was leaking the
password's actual length.
Adds secure SE credentials for jeremybanks/ChatExchange for Travis.
Modifies Travis icon link in README to point to /branches page.
Merge branch 'more-travis-credentials' into test
Merge branch 'hiding-passwords' into test