base: Add PakfireContext

base: Add PakfireContext

To make initializing Pakfire easier, this patch introduces
PakfireContext. So writing code like

  with Pakfire() as p:
      p.some_action()

will automatically take care of initializing Pakfire and
cleaning up afterwards.

Actions that do not require initialization will remain
in Pakfire so that they can be accessed as

  p = Pakfire()
  p.some_other_action()

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>