…docs - in docs/conf.py, use dynamic values in subs to save manual updates - Fixes #221
The current docs show `colander.DateTime(default_tzinfo=<object object at 0x7fef11f71aa0>)` due to using the `_marker`. `iso8601` has a `UTC` object that is an already instatiated version of `iso8601.Utc()` and outputs its repr as "<iso8601.Utc>". This should change the docs to print out: `colander.DateTime(default_tzinfo=<iso8601.Utc>)`.
When I was trying to introspect schemas I printed out "missing: <object object at 0x7fa8787da240>", "missing: <colander.null>" and "missing: <colander._drop object at 0x7fa8737e4978>". The first is supposed to be a colander.required but unfortunately it's just a generic object placeholder. This PR is to unify colander.required, colander.drop and colander.null.