Refacor Piece#current_version

Piece#current_version always returns the last version.
Issue #47 - Stop chance of duplicate version nums

A new version's number is set based on the last version's number,
instead of being set by the number of version's that piece has. This
removes the possibility of duplicate version numbers if an older version
is deleted.
Issue #47 - Stop chance of duplicate draft numbers

Add Piece#create_draft to handle creating drafts for a piece.
Refactor DraftsController#create to use this method.
A new draft's number is based on the last draft's number, instead of
being based on the number of drafts that piece has. This removes the
possibility of duplicate draft numbers if an older draft is deleted.
Fix #47 - Fix version/draft dup numbers bug