Add chdir parameter to pip module

pip's requirements.txt supports specs of the form:
```
-e ./some/relative/path
```
FOr this to work, pip must be run from the same directory as the requirements file,
not currently possible with ansible afaict.

I'd like to be able to deploy using ansible, or directly to heroku via push, 
but currently the same requirements file can't work on both, unless I switchto git submodules or git repos over net, which I'd like to avoid.

With that in mind, I added a `chdir` arg to the pip module, identical to that found in the `command` and `shell` modules.

Related: [Heroku devcenter](https://devcenter.heroku.com/articles/python-pip#local-filebacked-distributions) .