New user javascript validation.

JavaScript creates divs and inserts them with error messages
telling the user about what is required or missing. For now,
only a name, email, and password is considered really required.
Should it be necessary, it would be easy to add the same
validation to other fields.

The JStools.js has some useful functions, it is like a small
library. main_script.js has a lot of stuff, but it also has some
functions that don't belong in JStools.js. The validation of the
new user form itself is at the bottom of views/users/new.html.php
file. Not sure if that is the best approach...