Initial implementation of RestrictAccessMiddleware, to restrict the acce...

@antoviaque : This is a pull request for the recruitment task: Restrict access to the LMS by IP address.

This pull request adds the ability to restrict the access of the LMS by ip addresses or networks. I've added some documentation in the middleware implementation file. Tests are also implemented.

@jtauber : I've added a package in the requirements: ipaddr (http://code.google.com/p/ipaddr-py/), which is to manage ip addresses and networks. It is licensed under Apache 2. Let me know if there is any issue.

@e0d : A new package has been added in requirements for the middleware: ipaddr (http://code.google.com/p/ipaddr-py/). However, I don't think it needs any system requirement. 

Don't hesitate if you have any question or if there are any modification that I should do.

Best Regards,
Alan