Make public BelongsToMany::updateExistingPivot and BelongsToMany::newPivotStatementForId

I moved two methods to public : `updateExistingPivot` and `newPivotStatementForId` – those methods are really useful for working with pivot table attributes but they're protected, and `newPivotStatement` is already public so I thought it'd be a good idea.

I pointed this to master since it's not a bugfix or anything, not sure if that'd be better to 4.0 ?