feat(ngMessages): provide support for dynamic message resolution

Prior to this fix it was impossible to apply a binding to a the
ngMessage directive to represent the name of the error. It was also
not possible to use ngRepeat or any other structural directive to
dynamically update the list of messages. This fix ensures that both
the ngMessage and ngMessages directives automatically update when
any dynamic message data changes.

BREAKING CHANGE:

The ngMessage directive now uses expressions. Therefore, all
pre-existing ngMessage attribute values (as well as the values
assigned via the `when` attribute) will not function as they did
before because ngMessage will evaluate the attribute value as an
expression. A quick fix for this is to wrap single quotes around each of
the attribute values:

```html
<!-- AngularJS 1.3.x -->
<div ng-message="required">Your message is required</div>

<!-- AngularJS 1.4.x -->
<div ng-message="'required'">Your message is required</div>
```

The `ngMessagesInclude` attribute is now its own directive and that must
be placed as a **child** element within the element with the ngMessages
directive. (Keep in mind that the former behaviour of the
ngMessageInclude attribute was that all **included** ngMessage template
code was placed at the **bottom** of the element containing the
ngMessages directive; therefore to make this behave in the same way,
place the element containing the ngMessagesInclude directive at the
end of the container containing the ngMessages directive).

```html
<!-- AngularJS 1.3.x -->
<div ng-messages="model.$error" ng-messages-include="remote.html">
  <div ng-message="required">Your message is required</div>
</div>

<!-- AngularJS 1.4.x -->
<div ng-messages="model.$error">
  <div ng-message="'required'">Your message is required</div>
  <div ng-messages-include="remote.html"></div>
</div>
```

Closes angular#10036
Closes angular#9338

Author: matsko