Support for ActiveModel::Serializers (WIP)

Support for ActiveModel::Serializers defaults as discussed in #114

I've added `Her::Model.extract_array`.

This is not yet ready to merge. I've marked one spec (for `custom_get`) as pending as it seems not to treat the response data as an array. I'll look at this later.

Would also be great to get feedback on the general approach as I'm not entirely happy with it.