move dispatch into isso package
isso.dispatch now dispatches multiple sites based on relative URLs

The previous approach using a custom X-Custom header did work for the
client-side, but not for activation and deletion links. Now, you need
to add a `name = foo` option to the general section. `isso.dispatch`
then binds this configuration to /foo and can distinguish all API
calls without a special HTTP header.
move application export to isso.run

When using Gunicorn or uWSGI to run `isso.dispatch` it would
automatically initialize and a default Isso instance (and cause
several logging messages), although never used.

If you use uWSGI or Gunicorn, you have to change the module from
`isso` to `isso.run`.
Merge branch 'fix/multsite'