Simplify the queries passed into init

Instead of using an array of objects to define the queries I've changed it use an object. The key of the object is the context (skinny, mobile etc) and the value can be either the callback function or an array of callback functions.

Internally all the callbacks are kept as an array against the context key - this means that ```addQuery``` just appends another callback onto the context.

I think it makes for a nicer API - but that's just my opinion!