Adding saveStrategy to BelongsToMany for handling whether to append or
replace to the junction table
Adding tests for saveStrategy
Adding unit tests for BelongsToMany::save() when combined with the
replace strategy
Adding docblock for saveStrategy
Fixing problems found in review
Makin replace the default strategy for saving belongsToMany, this
revealed some inconsistencies between both strategies which are now
solved
Merge pull request #2500 from cakephp/3.0-habtm-strategies

3.0 habtm strategies