Add helper for Meta.fields definitions for User forms

This is necessary because the Meta def itself is sufficient to crash the
app for a different User model - the form doesn't even need to be
instantiated. There is still plenty more code that makes explicit
reference to first_name and last_name however.

Fixes #1275

Moved to oscar.core.compat and renamed by @maikhoepfel
Extend tracker pixel to include Python and Django version

Reasoning about e.g. when it is feasible to drop Python 2.6 or Django
1.5 support is hard without reliable data, hence the tracker pixel has
been extended to submit the Python and Django version in use. Tracking
is still easily disabled by setting OSCAR_TRACKING to False.
Use existing_fields filter for UserForm, and use correct User model

I messed up when moving Alex Moon's code to oscar.core.compat and used
the wrong User model, defeating the point of the exercise.

This is fixed now, and the filter is also used for the UserForm that
gets used in the account section.

Fixes #1283.
Fixes #1282.