feat($parse): revert hiding "private" properties

Hiding `_*` properties was a feature primarily for developers using Closure compiler and Google JS
style. We didn't realize how many people will be affected by this change.

We might introduce this feature in the future, probably under a config option, but it needs more
research and so I'm reverting the change for now.

This reverts commit 3d6a89e8888b14ae5cb5640464e12b7811853c7e.

Closes #4926
Closes #4842
Closes #4865
Closes #4859
Closes #4849

Conflicts:
	src/ng/parse.js
Merge branch 'master' of https://github.com/angular/angular.js
componentizd!