switch to multi_json

[xn]

This switches out json_pure for multi_json. json_pure stops some rubyists from utilizing your service because it is incompatible with other json libraries they might be using. This is a drop in replacement and no loss of functionality or speed will occur for existing users. Furthermore, customers using other json libraries like oj will have a marked speed increase.

Also, I added a Gemfile so I could easily install your gems, fixed a broken test and edited a redundant line of code.

Thanks for considering this!