Refactor Piece#user specs
Add Piece callback create_first_version
Refactor Piece#versions specs
Refactor piece#draft specs
Refactor Piece#current_version and specs
Refactor Piece#title and specs
Refactor Piece#title= and specs
Refactor Piece#short_title and specs
Refactor Piece#content and specs
Refactor Piece#content= and specs
Refactor Piece#blurb specs
Make Piece#changed? specs more unique
Refactor Piece#current_version spec
Refactor Piece versioning and specs

Piece manages versions more completely and properly.
Piece creates a version if it doesn't already have one.
Title and content use a default as necessary.
If a piece is being modified, it creates a new version.