Why view_shortcuts.filters, not django.contrib.admin.filterspecs?

- ordinary views, not only admin
- lookups mapped to params --> security
- no pre-generated html --> valid, flexible
- QuerySet in, QuerySet out: can pre- and post-filter
- choices with respect to existing QuerySet

What's good in filterspecs (included here now, too):

- factory with simple tests
  - enables field-specific behaviour
  - easily extendable with custom filterspecs

TODO:

- more filters:
  - alphabetic
  - date drilldown
  - date fadeout
  - range selection
- add ability to enforce a filter when creating a FilterList
  (e.g. date drilldown/fadeout have to pass the same test and, therefore,
  the one which is declared earlier will be chosen automatically; this makes
  no sense and developer should be able to choose the field manually.)
