different buy/sell prices for commodity and equipment

This is work in progress, but I might as well put it here as a semi PR/feature request. I'd like to get some input.

It seems pretty much agreed upon (from discussions on IRC) that we should have different buy and sell prices for commodities and equipment. Plus I need it for my second hand ads I've done for the BBS, since otherwise you'd always buy anything sold on the BBS, and sell it back in the shipyard for full price.

The way I've done it now, the buy back price is always 80% of the full price. As has been suggested it could be different for different stations, but then we open up the can of balancing yet another part of the game. For me, having a fix lower buy-back price is, initially, good enough.

As is evident from the screenshots below, *if* one wants to display buy/sell prices, there will be a shortage of space (thus I did the #2774 to pave the way for this PR).

![foo6](https://f.cloud.github.com/assets/619390/2245010/232a6b68-9d48-11e3-8549-69c6d8409290.png)
![foo5](https://f.cloud.github.com/assets/619390/2245014/29bde4e6-9d48-11e3-8733-c0b27ab9a2b2.png)