Add Axiom::Tuple#to_hash
Add Axiom::Tuple#inspect
Add backtracing to rspec failure output
Change group/ungroup to nest/unnest to avoid confusion with SQL
Rename variables in join to be more consistent

* This is in preparation for extracting the code into something else
  to handle join and nest operations in a single class.
Move require statement for operation together with other operations
Add Axiom::Attribute::Relation#initialize and #header

* Ensure the header is passed in as an argument.
Add Axiom::Attribute::Relation#new_relation