Allow a password to be callable (e.g. a secretbox abstraction)

[sdvillal]

Could there be interest on a small new feature allowing passwords to be callables instead of the password itself? If so I can polish this PR.

[miketeo]

I have taken a quick look at your changes. It looks like it's doable with minimal impact on the API. Will you be able to checkout and submit PRs against the dev-1.1.x branch? The master branch is currently tied to the dev-2.0.x branch where the development is stuck at the SMB2.1/SMB3 protocol implementation.

[sdvillal]

It is indeed a minimal change.

I have open a PR for the 1.1.x branch #136.

I have added a short note to the docs.

Regarding tests, I feel it would be enough copy any connection test in in test_auth and, instead of passing the password, pass a function returning the password. I have pushed such test in the two PRs. Since I have not setup the infrastructure to run directly the tests, feel free to tell me or edit directly to my branch if they do not pass.

[sdvillal]

@miketeo Should we merge or close this too?

[miketeo]

If it's ok, I want to delay this merge as it is against the master branch. I have marked this PR as todo. When the changes are ready on dev-2.0, I will merge the branch into master and then merge in your PR.