Documentation Fix: Art.sy

Art.sy lost it's .sy domain and is now at http://artsy.net
Merge pull request #2146 from JasonPunyon/patch-2

Documentation Fix: Art.sy
Fixed comment for Model#_validate

The comment needed to be updated for the new "invalid" event type in the implementation.
Merge pull request #2148 from quartzmo/patch-1

Fixed comment for Model#_validate
Tidy up event internals, delete empty event keys in `off`, return `this` early in `stopListening`
Fix document for `Model#validate`
Merge pull request #2166 from hokaccha/fix_validate_doc

Fix document for `Model#validate`