Update writing_modules documentation: CasperJS in now able to call/load nodejs modules

The same way phantomJS can.
Add a better sample of nodejs modules integration https://github.com/n1k0/casperjs/pull/587
Update writing_modules.rst
Update writing_modules.rst
Update writing_modules.rst,"n1k0 compliant" :)
Merge pull request #587 from mickaelandrieu/master

Update writing_modules documentation: CasperJS in now able to call/load ...