Turbogears2 migration: replace pylons.url by kallithea.config.routing.url

In preparation for the migration to Turbogears2, introduce a
kallithea.config.routing.url to replace pylons.url. The implementation is
basically the same: wrap around routes.url().

This change involves:
- a number of import statement changes
- fixing some tests in test_libs.py; to avoid duplication, the different
implementations of fake_url were grouped in one place.

This change was first proposed by Alessandro Molina in his initial
port. Following changes were made afterwards:
- move UrlGenerator from kallithea.lib.utils to kallithea.config.routing
- add documentation to UrlGenerator
- kallithea/lib/auth.py used url_for instead of url, for no apparent reason
so this was changed.
- fix libs tests
- rebase onto Pylons-based Kallithea first