Check correct columns are present in join table

The `have_and_belong_to_many` matcher could give a false positive if the
join table is present but does not contain the correct columns. Check to
see if the columns exist in the join table and provide a meaningful
failure message if one or more of the columns are not present.
Join table check for habtm w/ custom foreign keys should still work

If the habtm association in question specifies a custom foreign_key or
association_foreign_key, when the habtm matcher checks that the join
table has the correct columns, it should use the specified column names
instead of the defaults.
Add NEWS for #511