Support setting passwords in get_shared_link()

The Box API [supports setting passwords on shared links](https://box-content.readme.io/reference#create-a-shared-link-for-a-file) but this feature is as yet missing from the shared link implementation in box-python-sdk ([here](https://github.com/box/box-python-sdk/blob/master/boxsdk/object/item.py#L162)).

I'm currently working up a pull request for this enhancement.
