Add `Document.documents` method to list user docs

This pull request fixes:
- #2 (7fda269113b2d004855fd6ed88d0075fbc42d2e8)
- #24 (0de9994d17488ac5fe3cef37395cb09b94c98386)
- #23 (5a99c493b6d17794a8a4c2291258ba46b1bdf4ee)

Note: there is a typo on commit message (7fda269113b2d004855fd6ed88d0075fbc42d2e8): it changed `PyPLN.documents`, not `Document.documents`.