use activerecord query interface instead of deprecated finder methods

These changes are Rails3 and Rails4 compatible and necessary if Lobsters will ever run on Rails4 without the `activerecord-deprecated_finders` gem, and later Rails5.
For the most part, they change `find_by_*` finder method calls to `where` calls.

You will also find a working `rails4` branch in a separate branch of my fork. You could consider testing and merging that as well, if you wish.