Document stand-alone usage

[knownasilya]

Fixes #134

[alexlafroscia]

If we're going to mention this in the readme (and I'm worried out readme is getting really bloated) we should do both request and raw, and explain the difference.

[knownasilya]

@alexlafroscia @rwjblue how is this?

[alexlafroscia]

Great! Can you just add a little usage example below each import? Something like

import request from 'ember-ajax/request';

return request('/foo', { ... });

and the same for raw. Just in case someone doesn't know what to do once they have the function.

[knownasilya]

Added examples

[alexlafroscia]

Thank you!