default constructor should return parent value

I came across this issue with a class that caches instances when
they are firste created. Subsequent calls to the constructor will
return the cached instance if available. This relies on a feature
of JavaScript described here:

http://bclary.com/2004/11/07/#a-13.2.2

Note Step 7 in the reference document. By not returning the value
from the default constructor, subclasses of Backbone classes that
rely on this feature will break.
Fix #1939 - `parse` receives `options`
allow context as 4th parameter to Events#listenTo method
allow context as 4th parameter of stopListening
updated docs to add context param in listenTo and stopListening
accounting for event maps
fixed typo on context var in stopListening
Merge pull request #1951 from caseywebdev/parse

Fix #1939 - `parse` receives `options`
Merge pull request #1947 from cdavaz/master

default constructor should return parent value
Improve getting of models with non-default ids

Just as you can get a model from a collection using default ids by passing .get() an object i.e. col.get({id: 1}), you can now do the same when using non-default ids i.e. col.get({_id: 1}) (If the collection prototype has the correct model set).
Make use of new .get() functionality in .update()
Fix #1925 document for {sort:false}
Merge pull request #1956 from wyuenho/master

Fix #1925, Document for {sort:false}
Fix getting model from collection by model

Previously wouldn't work if using non-default id. Thanks to @caseywebdev for pointing this out.
fixing docs on validate
Merge pull request #1957 from tgriesser/validateDocs

Fixing docs on validate
Merge pull request #1955 from johtso/get-by-idattr

Improve getting of models with non-default ids
Merge pull request #1954 from derickbailey/listenTo_stopListening_context

add context as 4th parameter to listenTo and stopListening methods on Backbone.Events
Simplify internal id/cid split. Add back in update-with-nonstandard-id test.
Merge branch 'master' of github.com:documentcloud/backbone