Auto-detect currency locale

Oscar is slowly adding better support for switching languages on the
fly. That also means that locales need to be switched.

The issue this commit tries to fix is that when switching languages, the
locale for rendering prices doesn't change, and hence prices are
formatted incorrectly.
Oscar used to require hardcoding the locale for prices via
OSCAR_CURRENCY_LOCALE. I'm not sure what the reasoning behind this was,
because as far as I'm aware, the locale can be auto-detected from the
current language. This not only reduces complexity, but nicely works
when switching languages.
pylint fixes

pylint in Ubuntu 14.04 seems to have added a new style check.
Usability fixes for model-based shipping methods

Those tweaks make managing shipping methods in the admin significantly
more usable, and are a quick fix till a dashboard interface is
implemented.

* Adds default ordering by name for shipping methods
* Orders weight bands by method first, weight range second
* Adds a nicer interface to add/remove countries in admin
Refactor out a reusable widget for AJAX selects from ProductSelect

ProductSelect is a handy widget that can easily be re-purposed for other
AJAX lookups. This only required offering a general purpose way to pass
the lookup URL for the AJAX calls.

Should have no implications for backwards compatibility.