added support for with statements in open and creat

when creating or opening a File, caller should now
use python's with statement as specified on PEP 343:
http://www.python.org/dev/peps/pep-0343/

Change-Id: Id98613e310bea01a427769d3ad7b0dea97245b04
Signed-off-by: Thiago da Silva <thiago@redhat.com>