Add backend for dgraph database

[oubiwann]

This is going to be a little tricky, since there's no Erlang driver. We can start with an HTTP driver for the database.

Depends on:

• https://github.com/lfex/dgraph/issues/1
• https://github.com/lfex/dgraph/issues/2
• https://github.com/lfex/dgraph/issues/3

Resources:

• https://github.com/dgraph-io/dgraph
• https://dgraph.io/docs/clients/raw-http/

This closes:

• 39