Update Guardfile comments
Remove global values from gemspec

* Update the gem license to be a string
Remove unnecessary code from spec_helper

* This is all provided by devtools, so it is redundant
Add nested relations

* This branch will add support for nested relations. It will add
  #nest and #unnest operators to Axiom::Relation. It should add an
  Axiom::Relation attribute type. It should also propagate
  materialization, insertion and deletion across nested relations.