remove dependency "invoke"

The python-Package "invoke" is used in several places on the code. As far as I see on a quick glimpse, it does nothing that subprocess wouldn't also do.
The difference is that subprocess is tested and integrated into python core. Invoke has several issues that catch my attention (being alpha stage with explicit warnigns that API might change at will, emulating far more than is sane for something running inside python (sudo!?)).

Also, on an install with non-pip methods, I would need to manually package invoke.
I heard that up to now, ringo worked without having invoke installed, which is a mystery to me, as on my test install, it didn't work without.

edit: See also #74 (PR for this issue)

[toirl]

Indeed using invoke is some kind of convinience as in early phases of ringo invoke played a more important part in automatic things like setup etc. Other based ringo projects stlll use invoke in this way. Also Ringo has a usage for this but it is not recomended to be used, not documented in the sense that things which can be done using invoke are documented how to to this things step by step without invoke.

So I appreciate to remove the dependecy from invoke by replacing it with subprocess calls. However we make sure that other application does not break.