use SimpleDelegator 

Hi there, 

I am making 2 small changes in order to fix some problems I had with the library.

1) I am changing the `MultiMail::Message::Base` class to inherit from `SimpleDelegator` instead of inherit from `Mail::Message`. The reason behind this is that when an instance of `MultiMail::Message::Base` is created, the original `Mail::Message` is serialized into a string and then parsed back into another `Mail::Message` object. Apart of being more performant, this fixes a problem I was having with the encodings. 

If you have some `UTF-8` characters into the html part, when serialized is then converted into an `US-ASCII` string, and when parsed again and converted into a `Mail::Message` object, the html body is converted into a `7bit` string.
If you then call `JSON.dump` and you pass that string, it fails saying it can't convert that unknown `7bit` character into a `UTF-8` one. Conserving the original `Mail::Message` solves this

2) In `MultiMail::Sender::Base` I am changing the `attr_reader` for `@settings` into an `attr_accessor`, to maintain compatibility with the original `delivery_methods` of the `Mail` gem.

Thanks