Support nested attributes in create/update requests

Well the title says everything.

I've found that there is a Sequel [plugin](http://sequel.rubyforge.org/rdoc-plugins/classes/Sequel/Plugins/NestedAttributes.html) designed for nested attributes, now I've got to make it work while still learning ruby so that kind of a challenge :) 

I will try to submit a PR in a separated branch if I manage to make it work !