Implement comparison methods

[SivanMehta]

This PR implements the compareHeight and the compareWidth methods. Here is the documentation added to the README that describes the usage of these methods:

compareWidth(breakpoint)

Returns the difference between the current width and the given breakpoint. This can be used to check if the window is "greater" than a breakpoint. If the given breakpoint does not have a width, this will always return the current width. If the given breakpoint does not exist, than we return an error.

var breakpoints = new Breakdancer([
  {
    name: 'wrist',
    width: 320
  },
  {
    name: 'palm',
    width: 800,
    height: 600
  }
]);

// let's assume the window 500 is px wide.
console.log(breakpoints.compareWidth('wrist')) // 180
console.log(breakpoints.compareWidth('palm')) // -300

compareHeight(breakpoint)

Same usage as compareWidth

TL;DR

Now you can check if you're "past" a certain breakpoint with compareWidth('mobile')

[Swaagie]

Mostly looking good, seems like a solid feature to add.
Would like to have eyes from @3rd-Eden

[Swaagie]

might want to assert the functions are called with the right property e.g. width and height

[Swaagie]

wonder if we should safeguard against this, although both native and regular have default implementations.

[SivanMehta]

I was thinking that because this is already a @private method, we can safeguard this by relying on the fact this.height() and this.width() methods already exist in index.js and index.native.js.

However, it would be worthwhile to throw a ReferenceError if the associated this[property] function does not exist.

[3rd-Eden]

In all honestly, I would say you might as well just ditch compareHeight and compareWidth and make the compare method public and keep that as only API.