refactored little bit

.. in the continuation of the https://github.com/diefenbach/django-lfs/commit/acb8b8b5035beacdd6add7f64e14380cfd8fa7fc
typo in lfs.customer.views
Merge pull request #85 from baffolobill/patch-11

typo in lfs.customer.views
allow for longer e-mails (issue #19) - a missed field
Merge pull request #86 from baffolobill/patch-12

allow for longer e-mails (issue #19) - a missed field in the lfs.order.models.Order
Update models.py

Strings are immutable sequences
Merge pull request #88 from potar/patch-1

Update models.py
remove some raw sql statements in favour of django ORM - this way we should avoid some potential problems with memory usage and sql IN clause limits for a LFS instances with a big number of products
fixed issue #82
Merge branch 'master' of github.com:diefenbach/django-lfs
do not show empty property groups
fixes delete_column bugs in lfs_migrate.py

django.db.utils.DatabaseError: (1091, "Can't DROP 'invoice_country'; check that column/key exists")
code style changes, removed unused imports; added cart to cart_inline template
Updated CREDITS.txt. Added docs for CREDITS.txt
Merge pull request #90 from baffolobill/patch-13

fixes delete_column bugs in lfs_migrate.py
updated credits
minor cleanups and coding style updates
Merge pull request #80 from baffolobill/patch-7

refactored little bit
do not use builtin 'property' as variable name; be consistent with core_utils.atof usage; minor code style changes
added upgrade notes