Add support for /proxy URLs.

Pencilcode will bounce requests for applications (as long as the referer
is also pencilcode.net) using the url /proxy/URL.

This allows applications to request cross-origin resources (such as images)
without security issues.  (For example, a proxied request will not have
access to domain-specific cookies, and it will not have access to any
private network.)
Add /proxy/ forwarding to dev server.
Add test for proxy functionality.
Merge pull request #69 from PencilCode/nginxproxy

Add support for /proxy URLs.