Adding "auto_add" parameter to circuits.io.notify.add_path()

[kromg]

Greetings!
As mentioned on IRC, I'd like to add a parameter: auto_add with default: True to circuits.io.notify.add_path().

The reason is that I noticed that trying to add subpaths manually when they're created can cause event loss. Passing auto_add=True to pyintoify seems to solve this problem.

If you're ok with this, I can submit a PR.

[spaceone]

There are syntax errors introduced by this PR for python2. Could you have a look again. E.g:
https://travis-ci.org/circuits/circuits/jobs/427305635

E     File "/home/travis/build/circuits/circuits/tests/io/test_notify.py", line 31
E       def create(self, *targets, assert_created=True, dir=False):
E                                               ^
E   SyntaxError: invalid syntax

[spaceone]

I fixed those syntax errors.