Move loading/printing of config out of config.ru and support reload via USR2

* Same as r509-ocsp-responder, move the loading/printing of the config out
  of config.ru and into a newly created R509::CertificateAuthority::HTTP::Config
  class.
* Trap the USR2 signal to reload the config

Fixes #11.
Merge pull request #29 from reedloden/split-config-out

Move loading/printing of config out of config.ru and support reload via USR2