improve python2+python3 support

now both python2+python3 bindings can be build at once
use proper python2 for gen tool (fixes problem at arch)
use python2 in python2 binding

Add full cmake commando for debian stable

move Cache to ElektraCache.cmake

(python2 binding is still out of sync with cpp binding)

add_headers where missing
exclude ini from testcases, please fix in #58