Invalid literal for int()

Firstly, thank you for this library. It's seriously impressive and so useful!

I ran into an exception when working with the is_running function:

  File "quip.py", line 40, in main
    if is_running('update'):
  File "/Users/orf/PycharmProjects/alfred-quip-workflow/workflow/background.py", line 99, in is_running
    pid = int(file_obj.read().strip())
ValueError: invalid literal for int() with base 10: ''

The code is taken from the getting started guide and isn't doing anything crazy:

    if not wf.cached_data_fresh('documents', max_age=60 * 60 * 60):
        cmd = ['/usr/bin/python', wf.workflowfile('quip-update.py')]
        run_in_background('update', cmd)

    if is_running('update'):
        wf.add_item('Getting new posts from Pinboard',
                    valid=False,
                    icon=ICON_INFO)

I haven't dug into it, but it seems to me like it's a race condition where the file is created by the new process but the pid has not been written to it yet.

Using something like the pidfile library solves a lot of these issue, could you vendor it inside this library?