Add keys, values, pairs, invert, pick, omit, each, and other
underscore methods to models
Revert "Issue number in comments."

This reverts commit b890e3e2f2e3b31987a7822724b1123f0e03bd17.
Removing the 'graf about model.set validating.
Reverting #2106 -- satisfying lint.
Documentation Fix: as -> are
add reset collection memory tests
Merge pull request #2142 from JasonPunyon/patch-1

Documentation Fix: as -> are
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`
Add Events#oneTimeListenTo
Add tests for Events#oneTimeListenTo
Rename Events#oneTimeListenTo to Events#listenToOnce
Fix document for `Model#validate`
Merge pull request #2166 from hokaccha/fix_validate_doc

Fix document for `Model#validate`
documenting validationError
Merge pull request #2172 from tgriesser/validationError

Documenting validationError
Changed the year from 2012 to 2013.