support mixed http-01 and dns-01 challenges

[allanrbo]

This adds support for certificates that requires both the dns-01 and http-01 challenge. I think this is fairly common, when you want a cert that works for both *.yoursite.com as well as just yoursite.com. As far as I can tell, it's Let's Encrypt's ACME server that decides whether to give us a dns-01 challenge, or http-01 challenge. In the case of *.yoursite.com,yoursite.com, from what I observed it will first require a dns-01 challenge for *.yoursite.com, and then a http-01 challenge for yoursite.com.

Also tried to reduce the line count a little bit by pulling back out the keyauthorization = ...-lines that are common for both http-01 and dns-01. This gets the line count down from 225 to 217. Still quite a bit higher than diafygi's 198, and he has stated that one of the project goals is to stay below 200 lines. So this might be a show stopper.

Suggesting to rename --challenge-script to --dns-01-script, so its purpose is clearer, since it's only ever used for the dns-01 challenge. If a new challenges comes up in the future, the script calling signature will probably need to be different anyway.

Added --txtrecord to the signature of the script being called, for consistency.

Switched to using _cmd instead of subprocess.call, to try to better fit into diafygi's code style.

Added some instructions to README.md.