Let the user override the default location conf's directory.

By default, Dancer load the configuration and the environments files
from the application's location.  However, it's possible to specify a
different location by specifying the environment variables DANCER_ENVDIR
and DANCER_CONFDIR.

Closes #308.
Load the views and public directory from the correct place.

Dancer was trying to load the `views' and `public' directories from the
configuration directory. Until now this was correct because the
application's location and configuration's location were always the
same.  Now that it's possible to specify a different location for each
of them, we should try to load `public' and `views' from the
application's location (that was the default behavior in Dancer 1).
Another update to the changelog.
Remove unused method and change the environments_location.

The `_build_config_location' method is now useless in Role::Config, so
we can remove it.

The `environments_location' attribute has been updated to look in three
locations:
 * DANCER_ENVDIR
 * DANCER_CONFDIR/environments
 * application's directory
The role Config requires the 'location' attribute.

All code consuming the role Dancer2::Core::Role::Config need to provide
a method or attribute 'location' in order to build the path for the
various configuration files.
Merge branch 'pr/dancer-confdir' into devel

* pr/dancer-confdir:
  The role Config requires the 'location' attribute.
  Remove unused method and change the environments_location.
  Load the views and public directory from the correct place.
  Let the user override the default location conf's directory.
Update changelog.