Changes the output of msc_validate to rename the sell offer files

This change is completely cosmetic but something that has been bothering me since working with this project. The correct terminology for the Sell Offer transaction data is offer, this project so far has been using bid, which is the term for the other side (buy offers). It might seem small now, but I think its an important change and this pays down some technical debt early enough. 

Since you forked his main repo, I only made the changes relevant to engine's wallet (namely just the msc_validate script).

I have a additional pull request for the web wallet, and I have tested that this setup works, all that is required is a restart of the app (nginx, app.sh)