improve pid file handling to fix locks and better handle problems deleting a stale file
Fix bug in delete task endpoint

Assignment now have a uuid instead of an id
Merge pull request #158 from pyfarm/improve_pid_file_handling

Improved PID Lock File Handling
Merge pull request #161 from guidow/bugfix_task_delete

Fix bug in delete task endpoint
Fix startup when there is no pid file.

On Windows at least, when the pid file is missing, open() will raise
IOError, not OSError, which the code wouldn't catch.  As a result, the
agent could not start up when no stale pid file was accidentally left
over.
Merge pull request #162 from guidow/no_pidfile_startup-fix

Fix startup when there is no pid file.