Add experimental elixir Geef.Repository wrapper
Add odb and reference wrappers
Better elixir example in README
Rename the ref record to geef_reference

Remove the access hacks for elixir and make :geef_reference an elixir
macro as well.
Rename the object module to geef_obj

This leaves us geef_object as the record to match against.
Don't forget mix.exs
Keep an object's id in the record

We have the information there anyway, so don't make us allocate a new
binary every time.