Extract shared model examples; refactor Piece spec
Issue #25 - Refactor version instance method specs

Version's instance method specs are now organized first and foremost by
method, not context.
Issue #25 - Refactor Version validation specs

Refactor Version validations to be consistent with Piece.
Move part of #blurb and #short_title specs to be in the instance method
area.
Issue #25 - Refactor User instance method specs
Issue #25 - Refactor Draft instance method specs
Fixes #25 - Incorporate all model spec refactors