Make json escaping use the default ruby gem

This makes making a json template a bit simpler and more reliable. The
json escaping is delegated to the ruby 'json' gem.

I ran into an issue where my content contained characters that were not
escaped by the original feed.json implementation.
Merge pull request #1 from mrvdb/mrb/jsonescape

Make json escaping use the default ruby gem
Update README.md