Craft can be modded to have sell value.

Simply put - this change allows a mod to add a 'costSell' value to crafts:

This change was created and tested off today's trunk (SupSuper/OpenXcom:master)

here is an example mod that allows you a one month 'refund' when you 'sell' your starting craft.
<pre>
crafts:
  - type: STR_SKYRANGER
    costSell: 500000
  - type: STR_INTERCEPTOR
    costSell: 600000
</pre>
![craftsell](https://f.cloud.github.com/assets/4869342/779505/22a4c2b8-e9db-11e2-8b9e-a680a0062e90.gif)