Remove BitDeli badge, because it's broken and deprecated.

- Giant orange warning message on their website:
  "Service disruption: GitHub has recently changed the way images are served in readmes. This has seriously limited our traffic analytics features for repositories."
- They haven't even responded to my support request on Twitter, which seems to be the only way to contact them:
  https://twitter.com/kodewerx/status/468938537174585345