update validation routine

* No longer store validation details for subentities on the parent entity.  Instead the parent will have a .customError property set to true, and the child will store all validation details.
* This also changes the .validate() signature to break from the Backbone standard.  Now we pass an action name as the first property instead of the attributes to validate.  This keeps things a bit more DRY
* We are also moving the "entity sniffing" logic down into the validateOne method.
update readme
update coverage report path
moved to test/coverage/
add "full" test coverage to backbone.siren.validate
housecleaning
minor
add "validateOnChange" flag

when it is set to true, attributes get validated whenever the ui attempts to change them