Rewrite to use argparse

This commit mostly rewrites nginx-upstream-manager to use argparse
instead of optparse. This gives us the ability to have more centralised
command line parsing, better usages for free, and stricter validation.

I have also changed a few defaults from '__file__ + path' to use
either command line overrides, or be relative to the config file itself.
This is because I would like nginx-upstream-manager to be installed,
rather than a script that just lies around.
Add argparse to requirements.txt
Preserve the _file config option
Expand documentation on 'rotate' command
Makesure to reload cluster configuration when rendering
Use cluster.render, not cluster.save
Merge pull request #1 from metabrainz/argparse

Argparse