Added support for non-header bearer tokens via proxy around Koa request

[brownstein]

This is a fix for issue #30 - Koa requests return an empty string when a header is missing instead of oauth2-server's expected undefined. This PR wraps authorize requests with a proxy and overrides .get to rectify the issue.

I'm a little ambivalent about adding the node-proxy dependency given that Harmony proxies are supported in recent nodes, but I think its probably the most universally-compatible way to do this.