Add additional meta fields to DocumentType

[KodrAus]

Closes #301
Closes #302
Closes #308
Closes #313

Includes index, ty and id. This allows us to infer the values for these document type specific fields in high-level APIs taking a T: DocumentType instead of requiring them everywhere.

It's part of a long-overdue revamp of the DocumentType trait design.

[KodrAus]

Could also clean up the client methods by wrapping the document type in a separate method so that generics can be used:

client.document::<MyType>().get(33).send()?;

instead of:

client.document_get::<MyType>(Id::from(33)).send()?;

That would make it clearer how DocumentType can be used to simplify request building.

[KodrAus]

Will need to figure out what the docs should look like for this change to the document methods.

[KodrAus]

This also includes some movement towards supporting Elasticsearch 6.0, but we'll need to build out more document type integration tests to be confident that stuff is correct.

Once this merges we should push vNext onto master, polish up the docs, figure out #317, and then get it out the door!