Refactoring select where clause logic in preparation for where in clauses.
Implementing where in clauses in mysql selector.
Cutting out a method call in AbstractSelector.
Making it easier to read the start of MysqlSelector#getSQL().
Adding a test author entity for testing relations.
Extracting some logic from EntityTest methods.
WIP implementation of relations.
Adding Entity::getRelationDefinition().

This allows for a single point within the class to get a named relation.
By making the relation information public, selectors can use this
information for fetching relations.
Merge branch 'relations'