Proxy support about certes

[fszlin]

Not at the moment...
I think if I change AcmeHttpClient to public, you'd be able to pass in a proxied HttpClient.

Will it work for you?

[webprofusion-chrisc]

This would be useful for https://github.com/webprofusion/certify as well. We should really be setting the User Agent also (or at a least be able to append to it) as Let's Encrypt needs to know what clients are connecting so they can flag any issues they see with the correct project.

[XSBen]

@fszlin yes, I think it's good :)

Thank !