popular scope for crops
Added unpopular scope to crop
Set default plantings_count to 0 not nil
Removed unpopular scope (guess it really was unpopular)
Allow selection of crop display order (popularity or alphabetical)

Had some trouble with tests; apparently the counter_cache behaves
differently when it starts counting from 0, which meant that some tests
started failing unless you do @crop.reload to refresh the
plantings_count.  Weird, but oh well.
Tweaked FactoryGirl to use create_list
Merge pull request #376 from Skud/cropbrowse

Show most popular crops on "browse crops" page