WIP: Ignore enhancement

This deprecates the `:ignore_nil` option in favor of a more flexible `:ignore` option:

``` rb
class UserPresenter < Jsonite(User)
  property :email, ignore: :nil?
end
```

The one difference is that, unlike other blocks in Jsonite, the symbol/proc is executed by the property value, _not_ the resource. Other options:

1. If a symbol, just send directly to the property; if a proc, exec by resource?
2. Exec both by resource instead (makes `ignore: :nil?` not as easy, though)