Couchdbkit Datefix

My patch to fix far future and really old dates from failing in couchdbkit

I've recently started getting better at including "bad" data in my tests after a few hair pulling episodes. While testing my apps with really old dates and far future ones, I discovered that the way couchdbkit handles date conversions breaks on *nix machines. couchdbkit.schema.DateTimeProperty.to_json uses timestamps during datetime conversions. Quote from time library: -- The functions in this module do not handle dates and times before the epoch or far in the future. The cut-off point in the future is determined by the C library; for Unix, it is typically in 2038.

I was going to publish a hotfix, but Benoit Chesneau being as cool as he is has already committed it to head. See Fix