renamed VariableOperation- to ActionInterface

moved the "entry_vars" and "exit_vars" state options, to be concrete
members instead.
for the xml markup this means that you can now use "<entry_actions>" and
"<exit_actions>" below a state node
(https://gist.github.com/shrink0r/03a45af4fef58e243cf3)
the ActionInterface and it's implementations where moved to a new Action
subnamespace.
refs #33