Better Context

This is the PR that is resulting from the discussion in #13.

Currently it sets the `context` object to be an empty Hash, and updates the README to reflect the new assumption going forward.

TODO: 

- [ ] Allow passing `merge_context` options to embedded serializers.
- [ ] Automatically set `:parent_serializer` in the context of embedded serializers.
- [ ] Consider adding built-in behavior for `context[:excludes]`, which would default to `[]`.