python/PyView: properly reference count parent objects

Before applying this patch, the output of the following
code was just empty:

def foo():
  s = metakit.storage(...)
  v = s.getas(...)

  return v

v = foo()
metakit.dump(v)

That's because the storage and its data already had been
reclaimed by Python before returning the view.

Hence metakit.dump(v) only printed empty lines and a total
row count (which was correct, BTW).
Merge pull request #4 from avdv/fix-refcount-pyview

python/PyView: properly reference count parent objects