Transparently support $https_proxy and $no_proxy environment variables.

This is a proposal for dealing with https://github.com/mozilla/browserid-verifier/issues/13 in a mildly generic way, by implementing support for the $https_proxy and $no_proxy environment variables.  These are used by at least `wget` and the python standard library to control proxying behaviour so using them here seems at least semi-standards-compliant.

Thoughts?

Possible improvements might include:  add a config option to explicitly disable them; process them once at startup rather than string-munging for each request.