Add optional "opt-in" field on checkout

The main purpose of this pull request is to optionally display an "opt-in" field on checkout.

Screenshot of the new settings:

![](http://f.cl.ly/items/2V3b34202q0W0t2O3w2O/Screen%20Shot%202013-10-30%20at%2010.22.33%20pm.png)

Screenshot of the checkbox displayed on checkout: 

![](http://f.cl.ly/items/1p3a1M1R271m381U0i2q/Screen%20Shot%202013-10-30%20at%2010.22.06%20pm.png)

Fairly atomic commits, so each commit message should be enough explanation on what that commit was for. 

The main two commits are SHA: 5740f90c861d3450a8f61ed1d939541c893eb6d4 & SHA: 7dddab34553fa2084edc42e9d52cf2d3411d4433 the rest are just to change whitespace, fix notices, tweak markdown etc.