Section the accounts table view by endpoint

These sections make it easy to see which logins belong
to which github endpoints. It is not uncommon for someone
to have the same login on github.com as they do on an
enterprise github installation. Fixed a problem in
this situation where the sliding view would show data for
the first account listed if it had the same login as
the selected account. Also cleared out the user and
organization caches when the account is changed.
Merge pull request #204 from lmarlow/logins_by_endpoint

Section the accounts table view by endpoint