new module: sysctl to handle sysctl entry

Hi,

Following your recommandations in the previous pull request : https://github.com/ansible/ansible/pull/1788 and https://github.com/ansible/ansible/pull/1806 i now submit new version of the module.

This is my first module for ansible,
Thanks for documentation and existing module to help me to write new one.
Once this one is OK, i probably push more modules.

Please do not hesitate to tell me if something wrong.