proposed backslash-python API

Here is what I thought the backslash-python API should look like.

Where there is TODO: it's a discussion I have with you/myself about the correct way to go

NOTHING is set in stone. Don't hesitate to change the whole thing if you think it is needed.
(Where there is Yotam - it's because the original version was Yotam's - feel free to handle it as well :))


I'm taking some of it back. I was referring to 'attaching' various artifacts to sessions and tests. The current artifacts of interest are warnings (which are messages, possibly with file and line numbers), exception tracebacks and notes (which are free-form markdown text boxes which can later be added through the web UI as well). 

Now that I think of it those would be better off as separate methods, but they need to be the same for sessions and for tests (warnings can be attached to both, notes can be attached to both etc.).