quicklock fails ungracefully if .lock exists as a file

[rtphokie]

Code assumes no file or directory named .lock exists in CWD. Should check for that and provide a more graceful error message (e.g. ".lock file already exists, remove it or specify a directory for quicklock to write to)

Traceback (most recent call last):
  File "./myscript.py", line 78, in <module>
    singleton('somename')
  File "/usr/local/lib/python2.7/site-packages/quicklock/quicklock.py", line 51, in singleton
    with open(lock_file, 'w') as lock_handle:
IOError: [Errno 20] Not a directory: '/Users/myuserid/proproject/.lock/somename.lock'