On signature creation, tabs can be computed by backend

This is #3 rebased on master + changed the way signature tabs are computed.
In #3, DocuSignBackend accepts an optional dictionary of tabs. One problem is that there are 2 formats: one for the "tabs" dictionary, one for pydocusign's models (such as pydocusign.models.SignHereTab).
In this one, DocuSignBackend uses an internal `get_docusign_tabs(signer)` method that returns a list of suitable pydocusign tab model for signer. Default implementation returns empty list, i.e. the signer will manually choose and place the tab.