new logo, bootstrap theme, chart and table colors

I thought a dark theme would be more familiar for financial data like this, and would be a nice contrast from the other demos.  I tried a jet-black theme called cyborg (which I've included), but the text was hard to read so I went with this "slate" theme which is a very dark grey.  I also customized the colors for bids and asks both in the charts and tables, and changed the TPS graph to green.  Also, I replaced the VoltDB logo.