Support for remote URLs for images

[SimonFairbairn]

1. Should check a local cache first which will populate the NSAttributedString instantly if the image is found
2. Otherwise starts a url request. On completion:
   1. If the image request is successful, caches the image
   2. Sends out a notification to give the user a chance to reload the string. If it was successful, the image will now be cached and will appear in the NSAttributedString.

[simonward]

Consider a configuration closure to handle loading of the image URL.

[NikitaSkripchenko]

@SimonFairbairn could you attach an example of this?