Shib PR responses to @cpennington and @ormsbee comments

* Changed unicode test cases to ascii encoding
* Removed 'stanford' hardcoding in TOS logic in lieu of
  'SHIB_DISABLE_TOS' MIT_FEATURES flag
* made 'external_auth' always an installed_app in lms
* log.exception changd to log.error where appropriate

But: did not change skipping tests to changing settings, for
reasons stated here:
https://github.com/edx/edx-platform/pull/67#issuecomment-19790330