Added dictionaries for numerical county codes and full county names.

I have added the following dictionaries and functions to se_counties:

NUMERICAL_COUNTY_CODES contains the numerical two-digit couty codes
which has more or less replaced the alphabetical codes nowadays. Also,
these numerical codes is always the leading digits in the codes for
municipailites and smaller subdivisions, which can be beneficial.

FULL_COUNTY_NAMES contains the full names of each county. The current
labels given in SE_COUNTIES is good when it is obvioius to the user
that he/she is selecting a county, but they are not identical to the
full county name. Also, in Swedish the county names differ somewhat,
e.g. "Skåne län" as opposed to the more generic "Stockholms län"
(ending with genitive case s), which the Swedish i18n should honor.

numerical_county_code() and full_county_name() are simply accessor
methods for the above dictionaries, returning the corresponding values
given a alphabetical county code.

For backwards compatibility I have chosen to keep both the
alphabetical codes and the shortened names (lacking the "county"-part)
in the original SE_COUNTIES list, although I guess one could argue
numerical codes and full names would be a better option.
Added @county_decorator() which adds a few accessor methods to models

The decorator adds get_FIELDNAME_numerical_code() and
get_FIELDNAME_full_name() methods to django models for all FIELDNAME
values supplied in the field_names tuple. Ideally this can be used on
all model fields that uses COUNTY_CHOICES for the choices argument.

@county_decorator(field_names('field',))
class MyClass:
    field = models.CharField(choices=se_counties=COUNTY_CHOICES)