Matthieu/add builtin file system image uplaoder resizer

Hi Jed

As I told you last week, I just pushed my work on github and here is the very first version of an FsImage fieldType + keystone.Uploader. 

Most of the code comes from cloudinary image example, 
and alleup node module (https://github.com/tih-ra/alleup) --> I removed all the useless code for our case and arranged it in a uplaoder.js file in keystone/lib/ directory. ( the guy doesn't answer to my pull request )

I don't know if my implementation matches the "keystone rules" (a feedback would be much appreciated!) but it would be great to include it in keystone. I will also implement a general file uploader to be able to get text files as well, and upgrade the uploader file manager for schema updates, managing multiple paths etc ... 

Also, i didn't really find a way to efficiently test my changes using keystone forked repo for a project, any tips ? 

I guess my pull request is quite stable , but i can't ensure this first version to work directly ( you know maybe I made some mistakes while copying the changes on the forked repo, but I'm pretty sure not :-)  ). 

Best, 

Matthieu