Use a unique "Acmebot" useragent

Created by: cpu

Hi there!

Thanks for writing an ACME client. I wanted to mention Section 5.1 of ACME Draft-04:

ACME clients SHOULD send a User-Agent header in accordance with [RFC7231], including the name and version of the ACME software in addition to the name and version of the underlying HTTP client software.

It doesn't look like acmebot is setting a UA right now, would you consider adding one? It certainly helps us from the server-side when we are troubleshooting client errors.

Thanks!