Ditch tracekit

This is based on `tracekit_window_onerror` branch, so diff is unusable. Better look at coffee files.

Summary:
- Switch to coffee
- Remove tracekit and resurrect fallback processor. Chrome, Firefox and Safari should be supported.
- And WindowError wrapper. Should be used like:

    window.onerror = function(message, file, line) {
      Airbrake.push(new Airbrake.WindowError(message, file, line));
    }