#115 add genre column to archives table

[Jayesh Sheth]

Add a genre column to the archives table. A genre can
act as an additional classifier, in addition to the
already existing category column.

Example:
if you have a blog with three primary topics, such as
technology, politics and movies, then there could be
three genres for each of these main blog topics.

If you already have a lot of fine grained categories,
these top level genres can be useful if you want to
provide an RSS feed for each genre, and for other purposes.