Adds a prereleases magic argument and update_settings option to enable prerelease updates

Created by: idpaterson

This pull request adds a prereleases magic argument and optional override in the update_settings dict which will allow users of a workflow to subscribe to a pre-release update channel. Tests are rewritten for py.test and changes are rebased on develop.

See #73 for discussion.

I am not confident on the tests for the workflow:update command and so left them in a separate commit for now. I ended up initializing the contexts with clear=False because the second test tried to run __workflow_update_check in the background which caused assert c.cmd == () to fail. I think that this is still a valid way to test since the update magic arg avoids the update info cache and does a clean update.check_update.

Whereas test_update asserts wf.update_available is False after creating that initial context, wf.update_available was True in the test_update_with_prereleases. Is the purpose of that initial run to get the background update check out of the way? I suspect something is leaking between these two tests, especially since assert c.cmd == () worked for test_update even without clear=False.

def test_update(httpserver):
    with ctx() as (wf, c):
        wf.run(fake)
        assert wf.update_available is False
    [...]

def test_update_with_prereleases(httpserver):
    with ctx() as (wf, c):
        wf.run(fake)
        # wf.update_available is actually True here
    [...]