Update hitcount/templatetags/hitcount_tags.py

Because hitcount using a models.TextField() for `object_pk` 
to allow *any* primary key type (integer or text), 
it can't use `unique_together` or `unique=True` to gaurantee
that only one HitCount object exists for a given object.

So lets check for exception and remove duplicates
Merge pull request #24 from inoks/master

check for exception and remove HitCount duplicates