Read the color pairs in top to bottom order, instead of prior complicated manner


![samplecolorsetup](https://cloud.githubusercontent.com/assets/271309/4828633/8ea59d30-5f81-11e4-97f4-8e21a15b667a.png)
![oldbehaviorvisualization](https://cloud.githubusercontent.com/assets/271309/4828637/91ba5d62-5f81-11e4-8b89-23fa6e30fd02.png)
![newbehaviorvisualization](https://cloud.githubusercontent.com/assets/271309/4828639/948c8588-5f81-11e4-9a65-0c80b92e238c.png)

This makes it so both g:rbt_colorpairs in the user .vimrc and s:pairs in
the autoload are easier to understand, with the highest level scope
being the first entry, the 2nd level scope being the second entry, etc.

The behavior this replaces is complicated and probably unintended, being a mix of truncation from max, extension from max, and using the list backwards. Note, I did not effect the s:max code, just the color order.