BREAKING CHANGE:
This commit introduces the notion of "private" properties (properties
whose names begin and/or end with an underscore) on the scope chain.
These properties will not be available to Angular expressions (i.e. {{
}} interpolation in templates and strings passed to `$parse`)  They are
freely available to JavaScript code (as before).

Motivation
----------
Angular expressions execute in a limited context.  They do not have
direct access to the global scope, Window, Document or the Function
constructor.  However, they have direct access to names/properties on
the scope chain.  It has been a long standing best practice to keep
sensitive APIs outside of the scope chain (in a closure or your
controller.)  That's easier said that done for two reasons: (1)
JavaScript does not have a notion of private properties so if you need
someone on the scope chain for JavaScript use, you also expose it to
Angular expressions, and (2) the new "controller as" syntax that's now
in increased usage exposes the entire controller on the scope chain
greatly increaing the exposed surface.  Though Angular expressions are
written and controlled by the developer, they (1) typically deal with
user input and (2) don't get the kind of test coverage that JavaScript
code would.  This commit provides a way, via a naming convention, to
allow publishing/restricting properties from controllers/scopes to
Angular expressions enabling one to only expose those properties that
are actually needed by the expressions.

Closes #4503

It is instructive to give literal examples that reflect common (my) experience of the problem. :)

Closes #4449

Closes #4418

This also contains some whitespace corrections by my editor.

Objects received from outside AngularJS may have had their `hasOwnProperty`
method overridden with something else. In cases where we can do this without
incurring a performance penalty we call directly on Object.prototype.hasOwnProperty
to ensure that we use the correct method.

Also, we have some internal hash objects, where the keys for the map are provided
from outside AngularJS. In such cases we either prevent `hasOwnProperty` from
being used as a key or provide some other way of preventing our objects from
having their `hasOwnProperty` overridden.

BREAKING CHANGE: Inputs with name equal to "hasOwnProperty" are not allowed inside
form or ngForm directives.

Before, inputs whose name was "hasOwnProperty" were quietly ignored and not added
to the scope.  Now a badname exception is thrown.

Using "hasOwnProperty" for an input name would be very unusual and bad practice.
Either do not include such an input in a `form` or `ngForm` directive or change
the name of the input.

Closes #3331