refactor(belongs-to-many): initial work on belongsToMany as a replacement to 2x hasMany
refactor(belongsToMany): get association unit tests working for belongsToMany
refactor(belongsToMany): change the docs a bit and improve restriction handling
refactor(belongsToMany): change tests to only look at number of sql statements and not sql statement content
Merge pull request #2687 from sequelize/refactor-belongs-to-many

Refactor 2 x hasMany to belongsToMany