This updates BaseModel to accept functions in place of values for some

of the new static attributes, and to add missing attributes for

customization.
defaults can now be a hash or a function. In function form, it can

access this, since it's being copied to and run off of the prototype

at class construction time.
urlRoot has been added, which was missing before. This can be a string

or a function.
Declarations for instance-level defaults and idAttribute were

removed, as they weren't actually needed.
A new ModelAttributes type has been added and exported, allowing for

easier reference without referencing Backbone.ObjectHash.
Unit tests have been added to test all model construction behavior, with

static attributes or functions.

Unit tests pass.