Fixed Play application path in dist start script

Set the user.dir system property (ie, the working directory) to be the
Play application directory.

Fixes:
  #1574 - Better error reporting
  #1576 - Running PID gets set in predictable location
  #1577 - Application path not set
Merge pull request #1577 from jroper/dist-working-dir

Application.path points to wrong place