add additional edge-case unit tests for finding all services by all tags

Signed-off-by: Craig Furman <cfurman@pivotallabs.com>
rename method to be more consistent with existing methods

Signed-off-by: Craig Furman <cfurman@pivotallabs.com>
document new find_all_by_all_service_tags in README

Signed-off-by: Craig Furman <cfurman@pivotallabs.com>