Inheritable Trash tranforms

The problem:
```ruby
class Result < Hashie::Trash
  property :cached_until, transform_with: ->(v) { Time.parse(v + "UTC") }
end

class OtherResult < Result
  property :server_open, transform_with: ->(v) { v == 'True' }
  property :online_players, transform_with: ->(v) { v.to_i }
end

Result.transforms # => {:cached_until=> #<Proc: ... (lambda)>
OtherResult.transforms # => {:server_open=> #<Proc: ... (lambda)>, :online_players=> #<Proc: ... (lambda)>}
```
Properties reader was defined [here](https://github.com/intridea/hashie/blob/master/lib/hashie/dash.rb#L52L55)