Introduces the concept of a "feature" that can be hidden. A list of
features to hide can be supplied in OSCAR_HIDDEN_FEATURES.

Feature hiding is currently supported in two places.
Application instances can carry a "hidable_feature_name". If set and
found in OSCAR_HIDDEN_FEATURES, post_process_urls will return an empty
set of url patterns, hence disabling relevant views.
display_tags also has a new template block tag iffeature.
{% iffeature "feature" %}xxx{% endiffeature %} will hide everything
inside the block if the feature is hidden.

Fixes #614

It handles the situation where there is no profile model too, and
dyncamically builds a profile and user form

segments.

This ones won't causes pep8 to complain.