- Using xenolf/lego's likely-temporary acmev2 branch
- Cleaned up vendor folder a little bit (probably more to do)
- Temporarily set default CA URL to v2 staging endpoint
- Refactored user management a bit; updated tests (biggest change is
  how we get the email address, which now requires being able to make
  an ACME client with a User with a private key so that we can get the
  current ToS URL)
- Automatic HTTPS now allows specific wildcard pattern hostnames
- Commented out (but kept) the TLS-SNI code, as the challenge type
  may return in the future in a similar form