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 `git push`. Currently the same 
requirements file (with the above pattern) can't work on both unless I switch to 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) .

Glad to make any changes or add docs, if needed, to help get this in.