Update README.md

Fleshed out Configuration section
Changed Gemfile to source 'https://rubygems.org'.
Following warning:
The source :rubygems is deprecated because HTTP requests are insecure.
Please change your source to 'https://rubygems.org' if possible, or 'http://rubygems.org' if not.
Raise CommandFailedError when wkhtmltoimage doesn't exit successfully.
Checking for result.size == 0 may fail to raise even though wkhtmltoimage exits unsucessfully (it can output only a few new lines to result).
Merge branch 'master' of github.com:dukedave/IMGKit
Merge pull request #40 from dukedave/master

Updated README about options, raise CommandFailedError more aggressively

Now we check the exit status of the wkhtmltoimage to see if it was successful.