Adapt code to archive DMs

* Add argument to get DMs
* Adapt statuses_portion()
* Adapt statuses() to optionally handle DMs
* Adapt main() to pull down DMs if instructed
Allow storage of date in ISO (RFC 3339) format
Convert archiver.py and follow.py to API 1.1

This is mostly done. I've not yet decided on a tidy way to re-implement
the API limit tests, since this has changed significantly between API
versions 1.0 and 1.1.

Further, as I understand it, API 1.1 requires OAuth for everything, but
it is still an optional command argument which is off by default. This
should be fairly trivial to fix, but I've not yet done so.