Feature/no media type

This PR is aimed at making a simpler implementation of the serializer magic.
There is a lot of cleanup still to do specially in the specs but this was all done in a hurry so today you can take a look at this and tell me if I am deleting too much or not.

The changes are:
- Make serialization much easier 
- Do not include it into Representor to avoid polluting it, instead add a to_media_type method that will call it.
- Delete stuff related to be able to render in different media types like .json and .yaml. We may need it but not sure if should be done in the same class versus specialized classes. In any case it can come in a PR later instead of the big PR we are reviewing these days

This works as expected:
```Ruby
format = 'application/hal+json'
document = {}.to_json
Representors::Serializer.build(Representors::Deserializer.build(format, document).to_representor, format).to_media_type
```
I think the factory classes are the easiest way to use this library and noone should know about the HalSerializer or HalDeserializer classes.

Things could be drier and more specs could be welcomed but tell your thoughts because maybe this PR generates some discussion

@fosdev  @sheavalentine-mdsol  @brutski please review , if you happen to like it, merge it. Further improvements can come later (this is a PR to Shea's branch)