switch to multi_json as dep

and only install oj if not using JRuby

refs #5
switch to multi_json as Oj was breaking JRuby

refs #5
switches to multi_json and fixes bug

Output to stderr if render was passed Array/Hash but request is not for
a json response. Thought this was better than silently swallowing the
response

refs #5
switches to multi_json and rescues all errors

if error class doesn't have http_status_code, then return 500.

This is still not great, since it could reveal implementation errors to
end users, but it's better than nothing at the moment.

refs #5, #10