[Fix #525] Change helm-projectile-sources-list order

- Prioritize file list above any other sources, except buffer since it is
usually small per project.

- Remove recentf list since it is just a subset of file list and if
  anything find-able in the recentf list, it should be inside file list.
  Remove to speed up helm-projectile command. User can invoke this command
  separately if they need a reminder of previously worked files.

- Remove directory listing for speeding up the command as well. Finding
  directory is a less frequent operation, and user can open a Dired
  buffer of the directory of current file anyway. This command has more
  meaning in stock Projectile then in Helm Projectile.

- Move project list to the bottom, since users won't need to switch
  project too frequently.