Fall back for systems without ack?

[thorsummoner]

My systems don't have ack installed by default; so I have two points:

1. If ack is not installed and uincaught OSError is raised:

Traceback (most recent call last):
  File "/home/myuser/.local/bin/todo", line 16, in <module>
    todo.interactive(match_names=args.names, show_count=args.count)
  File "/home/myuser/.local/lib/python2.7/site-packages/todo/__init__.py", line 77, in interactive
    matches = get_todo_matches()
  File "/home/myuser/.local/lib/python2.7/site-packages/todo/__init__.py", line 66, in get_todo_matches
    matches = subprocess.check_output(['ack', '--with-filename', "TODO\\(.*\\)"])
  File "/usr/lib/python2.7/subprocess.py", line 566, in check_output
    process = Popen(stdout=PIPE, *popenargs, **kwargs)
  File "/usr/lib/python2.7/subprocess.py", line 710, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1327, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

Suggest wrapping like 66 in a try catch to fail gracefully and alert the user that the programs depends on ack, link to readme w/e.

2. Allow for alternate/fallback get_todo_matches modules so pure python implementations can be used to not have a hard dependency on ack when not available.

Cool tool!