Add a hostname module, which sets system's hostname.

I send this pull request in response to
https://groups.google.com/d/msg/ansible-project/komqS4WdqHU/cRfWiMnCR3cJ

Currently implemented on only Debian, Ubuntu, RedHat and CentOS.
Unfortunately I'm not familiar about settings hostnames on other systems.

I appreciate your feedbacks.

By the way, I searched the mailing list for prior discussions about a module to set hostname, but couldn't find them.
https://groups.google.com/forum/?hl=ja#!forum/ansible-project
@mpdehaan Could you point me to the url of the discussions?