[Core] Updated Fog::JSON to use JSON gem by default

This revisits pull request https://github.com/fog/fog/pull/1034.

I have updated all references from MultiJson to Fog::JSON. Fog::JSON will attempt to use MultiJson by default and fall back to JSON if necessary. (As suggested by @geemus in  https://github.com/fog/fog/pull/1034)

In order to do this, I had to update the gemspec to require JSON rather than MultiJSON. I also added MultiJSON to the gemfile to aid with development.