Add a new rake task to clean up old, read stories

A new rake task has been added: cleanup_read. This will delete
the oldest 1000 read stories in the database. You can pass in a different
number of stories to delete as an argument to the rake task.

rake cleanup_read[50]
Change cleanup task to remove stories based on a cutoff date

The cleanup task now takes an optional "number of days" argument
and defaults to one week otherwise. It will delete read, unstarred
stories older than the cutoff date.
cleanup formatting
add readme section for cleanup task
make remove! a class method
change to query based on published date instead of created_at
add basic test
update readme; collapse specs; rename task
Merge branch 'pr/223'
Merge branch 'master' of github.com:swanson/stringer