missing overview documentation in .js files

[pixelzoom]

Related to #2 (code review):

• Documentation at the top of .js files should provide an overview of purpose, responsibilies, and (where useful) examples of API use. If the file contains a subclass definition, it should indicate what functionality it adds to the superclass.

Most of the .js files have no overview description. Their overview documentation looks like this:

/**
 * @author Thiago de Mendonça Mildemberger (UTFPR)
 */

[pixelzoom]

@ariel-phet asked me to mark this one as high priority, since it's important for on-going maintenance.

[tmildemberger]

Now all .js files have an overview documentation.