feat($sce): New $sce service for Strict Contextual Escaping.

$sce is a service that provides Strict Contextual Escaping services to AngularJS.

Strict Contextual Escaping (SCE) is a mode in which AngularJS requires bindings in certain
contexts to result in a value that is marked as safe to use for that context One example of such
a context is binding arbitrary html controlled by the user via `ng-bind-html-unsafe`.  We refer
to these contexts as priviliged or SCE contexts.

As of version 1.1.6/1.2, Angular ships with SCE enabled by default.

Note:  When enabled (the default), IE8 in quirks mode is not supported.
In this mode, IE8 allows one to execute arbitrary javascript by the use
of the expression() syntax.  Refer
<http://blogs.msdn.com/b/ie/archive/2008/10/16/ending-expressions.aspx>
to learn more about them.  You can ensure your document is in standards
mode and not quirks mode by adding `<!doctype html>` to the top of your
HTML document.

SCE assists in writing code in way that (a) is secure by default and (b)
makes auditing for security vulnerabilities such as XSS, clickjacking,
etc. a lot easier.

Here's an example of a binding in a priviliged context:

    <input ng-model="userHtml">
    <div ng-bind-html-unsafe="{{userHtml}}">

Notice that `ng-bind-html-unsafe` is bound to `{{userHtml}}` controlled
by the user.  With SCE disabled, this application allows the user render
arbitrary HTML into the DIV.  In a more realistic example, one may be
rendering user comments, blog articles, etc. via bindings.  (HTML is
just one example of a context where rendering user controlled input
creates security vulnerabilities.)

For the case of HTML, you might use a library, either on the client
side, or on the server side, to sanitize unsafe HTML before binding to
the value and rendering it in the document.

How would you ensure that every place that used these types of bindings
was bound to a value that was sanitized by your library (or returned as
safe for rendering by your server?)  How can you ensure that you didn't
accidentally delete the line that sanitized the value, or renamed some
properties/fields and forgot to update the binding to the sanitized
value?

To be secure by default, you want to ensure that any such bindings are
disallowed unless you can determine that something explicitly says it's
safe to use a value for binding in that context.  You can then audit
your code (a simple grep would do) to ensure that this is only done for
those values that you can easily tell are safe - because they were
received from your server, sanitized by your library, etc.  You can
organize your codebase to help with this - perhaps allowing only the
files in a specific directory to do this.  Ensuring that the internal
API exposed by that code doesn't markup arbitrary values as safe then
becomes a more manageable task.

In the case of AngularJS' SCE service, one uses $sce.trust (and
shorthand methods such as $sce.trustHtml, etc.) to obtain values that
will be accepted by SCE / priviliged contexts.

How does it work?
-----------------
In priviliged contexts, directives and code will bind to the result of
$sce.getTrusted(value) rather than to the value directly.  Directives
use $sce.parse rather than $parse to watch attribute bindings, which
performs the $sce.getTrusted behind the scenes on non-constant literals.

As an example, ngBindHtmlUnsafe uses $sce.parseHtml.  Here's the actual
code (slightly simplified):

  var ngBindHtmlUnsafeDirective = ['$sce', function($sce) {
    return function(scope, element, attr) {
      scope.$watch($sce.parseHtml(attr.ngBindHtmlUnsafe), function(value) {
        element.html(value || '');
      });
    };
  }];

This feels like too much overhead for the developer?
----------------------------------------------------

SCE only applies to interpolation expressions.  If your expressions are
constant literals, they're automatically trusted and you don't need to
`$sce.trust` them.  (e.g. `<div ng-html-bind-unsafe="'<b>implicitly
trusted</b>'"></div>`)  Additionally, `a[href]` and `img[src]` sanitize
their URLs and do not pass them through {@link ng.$sce#getTrusted
$sce.getTrusted}.  This significantly reduces the overhead.  It is far
easier to pay the small overhead and have an application that's secure
and can be audited to verify that with much more ease than bolting
security onto an application later.

What trusted context types are supported?
-----------------------------------------

Context            | Notes
-------------------}-----------------------------------------------------
$sce.HTML          | For HTML that's safe to that's safe to source into
                   | the application.  The ngBindHtmlUnsafe directive
                   | requires bindings to pass isTrustedHtml.
$sce.CSS           | For CSS that's safe to source into the application.
                   | Currently unused.  Feel free to use it in your own
                   | directives.
$sce.URL           | For URLs that are safe to follow as links.
                   | Currently unused (`<a href=` sanitizes urls and
                   | does not require $sce.isTrustedHtml bindings.
$sce.RESOURCE_URL  | For URLs that are not only safe to follow as links,
                   | but whose contens are also safe to include in your
                   | application.  Examples include `ng-include`, `src`
                   | / `ngSrc` bindings for tags other than `IMG` (e.g.
                   | `IFRAME`, `OBJECT`, etc.)  Note that
                   | `$sce.RESOURCE_URL` makes a stronger statement
                   | about the URL than `$sce.URL` does and therefore
                   | contexts requiring values trusted for
                   | `$sce.RESOURCE_URL` can be used anywhere that
                   | values trusted for `$sce.URL` are required.
$sce.JS            | For JavaScript that is safe to execute in your
                   | application's context.  Currently unused.  Feel
                   | free to use it in your own directives.