Change subclassing of LinuxAdmin to composition by including LinuxAdmin::Common

[jrafanie]

If all we need is the behavior from LinuxAdmin::Common, we should just include the module into our classes instead of using inheritance.

[Fryguy]

Note from #16:
The LinuxAdmin::Common was written as subclassing initially because it was complicated to use the mixin when everything was class methods. Now that everything is moving to objects, technically it shouldn't even be a mixin, it should just be another object...in this case, like a utility object. Then when you want to run something you would say something like Common.run or Utils.run or something.

[Fryguy]

See #9 for work that will probably be related to this change.