set up pep8/whitespace git-hook

and install script for activating in all submodules
give user option to commit even if there are formatting errors

I don't think it's a good idea for there to be no way to commit
something where there's a pep8 error that you think is justified

While I think it's good to bring it to the user's attention
it's ultiamtely up to the programmer to make that call
cp instead of ln git-hooks, so they don't change depending on branch
make cp interactive by default; add -f option to overwite all; add usage
Merge pull request #2344 from dimagi/git-hooks

Git hooks