Add a basic http proxy to deal with cross-domain requests

First version. A node.js application that receives the requests an proxies
them to the web conference server configured. Solves the problem of having to
modify the server to accept cross-domain requests: with the proxy nothing has
to be changed in the server.

refs #1111
Add documentation for the proxy

refs #1111
Update proxy output image and fix wrong reference to it

refs #1111
More small fixes in the docs for the http proxy

refs #1111