Ensure chunking 'where' clause handled separately

Fixes a bug where the migration:

Lhm.change_table :foo do |t|
  t.filter "where foo.bar = 'baz' and foo.baz = 'quux'"
end

resulted in select statements with the wrong priority.

Now, all user-specified where conditions will be put in
parens.
Merge pull request #57 from soundcloud/fix-compound-wheres

Ensure chunking 'where' clause handled separately