Updates the app script and website for the correct terminology

This updates the app to name the sell offer data files by there correct terminology. Previously they used the term "bid" which represents the other side (buy offers). It's a easily overlooked thing but I think it's important to fix this now. 

I have tested this in conjunction with the change to mastercoin-tools (https://github.com/curtislacy/mastercoin-tools/pull/1) and verified the site's functionality is unchanged.

Note: this does not fix the current issue on the sell offer page (http://blockchain-1.labs.engine.co/selloffer.html?tx=e3b77fa025afdd263d414cf8ed4f8e22471c244edfa0d70acdaa1e88f9640936&currency=TMSC) 
which I did not even notice until I started testing this change. I can look into this next.

The complete steps for integrating this change should be:
* Pull new mastercoin-tools into node_modules once PR1 is merged
* copy etc/nginx/sites-available/default to /etc/nginx/sites-available/default
* restart nginx
* restart app.sh (it will take some time as msc_validate.py writes the new files)
* remove bids/* and bids/* if you want