First pass of new project polling framework

- Added eventmachine and em-http-request gems
- Added basic task to start the polling daemon
- Added new project updater which uses eventmachine
- Simplify updating of dependent builds
- Default build_status_url to return nil instead of same url as feed_url
  when it is not needed
Do tracker updating in new poller

- Don't reason about tracker velocities, just always show the current
  velocity
- Rudimentary tracker parser for new poller
- Remove StatusFetcher
Add poll once mode to project poller

- Added a poll once mode for the project poller, so it can be started
  as a short lived job on Heroku. Heroku seem to have hijacked
  Delayed::Job so you can't launch long lived jobs.