Remove examples externs file

This PR suggests to remove the examples.js externs file and systemically use the square bracket notation to set properties on the scope. This has advantages over using an externs file:

* we do not globally prevent renaming,
* no more examples.js externs file to maintain
* systematic/consistent way to prevent renaming of scope properties (currently we use externs for the examples and the bracket notation in ngeo directives)

What do you think?