Add osx_defaults module

Hi, I created the osx_defaults module which set or delete the Mac OS X user defaults values (See [defaults(1) Mac OS X Manual Page]( https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/defaults.1.html ) for details).
Could you review this and hopefully merge this into the Ansible core?

Caveats: Currently only type bool[ean] and string are supported.
I do not plan to support other types myself since I do not need to use other types right now.
I home someone who needs other types will send pull request.

You can try this module with my [osx_defaults role at Ansible Galaxy]( https://galaxy.ansible.com/list#/roles/1457 ).

Thanks.