arrow

ARROW-1872: [Website] Minor edits and addition of YAML for versions

Hi, in this version I've:

* Added a YAML file for tracking versions and outputting related links etc.
* Done some minor cleanup on HTML and images
* Added a small paragraph about what Arrow is to the front page

Author: Justin Dunham <riboflavin@justindunham.net>

Closes #1483 from riboflavin/master and squashes the following commits:

9ffb5263 [Justin Dunham] fix responsive layout
fece5578 [Justin Dunham] update front page description
6174a5fb [Justin Dunham] small image cleanups
173ead2c [Justin Dunham] slight improvements to mobile and html layout
fe9007d5 [Justin Dunham] add version vars, small amount of copy, and clean up formatting slightly