tg2: move routing mapper initialization from RootController class definition time to instantiation time

Things happened to work before, despite unfortunate code executation at import
time. TurboGears 2.4 changed something, so actual serving works, but pytest
fails with:

collecting ...
...
data/env/lib/python2.7/site-packages/_pytest/config/__init__.py:463: in _importconftest
raise ConftestImportFailure(conftestpath, sys.exc_info())
E ConftestImportFailure: (local('.../kallithea/tests/conftest.py'), (<type 'exceptions.KeyError'>, KeyError('paths',), <traceback object at 0x7fb6679030e0>))

or more useful, if raising the real exception instead of obfuscating with ConftestImportFailure:

...
kallithea/tests/conftest.py:15: in <module>
from kallithea.controllers.root import RootController
kallithea/controllers/root.py:29: in <module>
class RootController(RoutedController, BaseController):
kallithea/controllers/root.py:31: in RootController
mapper = make_map(config)
kallithea/config/routing.py:32: in make_map
rmap = Mapper(directory=config['paths']['controllers'],
data/env/lib/python2.7/site-packages/tg/configuration/tgconfig.py:28: in __getitem__
return self.config_proxy.current_conf()[key]
E KeyError: 'paths'
_importconftest <_pytest.config.PytestPluginManager object at 0x7f20c770a3d0> .../conftest.py

In this example, in RootController, the global config object is at this time just:

{'debug': False, 'package': None, 'i18n.lang': None, 'tg.strict_tmpl_context': True, 'tg.app_globals': None}

Solved by moving the mapper initialization to __init__.