Add a wsgi support

[sakost]

I think that it will be gratefull, if this library will add a wsgi support, because for big projects it needs to use a web-servers, such as nginx or apache, because of a big load.
I suppose that the code from this:

tgflow.start()

Will be changed to this:

app = tgflow.wsgi_app()

And also this code will be in a separate module

This is also implies that lib has to have a callback support