Replace json codec with agnostic multi_json #1026

Some profiling stats (MRI19 on x64):

With ActiveSupport::JSON: load time 1.76, no laziness, memory 122Mb
With just MultiJson: load time 1.58, lazy 'json', memory 120Mb
With `gem 'yajl-ruby'` before `gem 'padrino'` : load time 1.59, lazy 'yajl', memory 114Mb
`oj` would be faster, but it uses the same amount of memory as stock `json`