Add a common health/status wsgi middleware

For those of us that use load balancers, or want an equivalent of 'mod_status' in python wsgi it would be useful to provide such a piece of middleware that can attempt to provide details (if configured) that are similar to that apache extension. This is useful for debugging, load balancing (noticing when a server craps out) and such.

Current thoughts:

1. Provide a configurable option which will enable more 'server' details (otherwise just respond with status 200 for HEAD requests for example)
2. If enabled provide basic eventlet information (not much is currently possible, but in the future it'd be neat to see pool size, green thread request time, and such, bytes transfered/recv...) and basic python system information (think phpinfo() for python), some basic platform information and the request environment.

Possibly like: http://paste.openstack.org/show/20834/