ESV Bible support

Hi!

I added support for the ESV Bible translation.

Since you targeted the web services API of the ESV Bible, I first looked there. Unfortunately their XML output is broken for some verses. Thus, I decided to use the RSS feed of the ESV's publisher site. You can find it at http://www.gnpcb.org/esv/share/rss2.0/daily/. It outputs the same daily verse as the web services API.

All tests pass, and the gem installation was successful.

I am looking forward to your response. :-)

Kind regards
Sebastian