Obtain certificate for acmeproxy using the acme-client mode

[fhofherr]

Naturally acmeproxy has to encrypt its own communication. Therefore we have to obtain a certificate for the public instance of acmeproxy using the acme-client operation mode.

Implementation note:

Upon startup acmeproxy checks if it already has a certificate for itself. If not it uses the self-test endpoint (#7) to check if the DNS server already has been configured correctly. Then it tries to obtain a certificate for itself. If the self-test fails acmeproxy waits for an configurable amount of time and retries to obtain a certificate.