Expose getEmValue()

I propose exposing the private **getEmValue()** method, essentially adding it as an API option.  I've updated the code to only do what's absolutely necessary to expose the method, then updated the documentation with a simple example.

I tested the updates with /test/test.html, successfully running respond.getEmValue() in the console.

Also, I've used this successfully to get [ResponseJS](https://github.com/ryanve/response.js) to play nice with [The Goldilocks Approach](http://goldilocksapproach.com/) ([Goldilocks demo](http://www.designbyfront.com/demo/goldilocks-approach/)), which uses EM media queries.

FYI, first time submitting a pull request and using github.