Operation hooks improvements

[fabien]

This is a combined PR - because of the expected conflicts/overlap of changes.

It implements the following improvements:

To maintain 'state' between before and after hooks, a hookState (object) property was introduced to context.

For consistency, all instance-level operations (prototype.updateAttributes and prototype.delete) will now have access to the current instance as part of the hook's context:

• prototype.save + before save: the instance will be available as instance (unchanged behavior).
• prototype.save + after save: the instance will be available as instance (unchanged behavior).
• prototype.updateAttributes + before save: because the instance should not be manipulated directly, the instance will be available as currentInstance to distinguish between the instance that can be directly modified (it is omitted now). In other words, currentInstance provides the current 'state' of the instance, and it should be regarded as immutable. Any modifications should be applied to context.data, which incorporates the partial nature of the updateAttributes operation.
• prototype.updateAttributes + after save: in after hooks, this difference is not important, and thus, the instance will be simply be available as instance.
• prototype.remove/destroy/delete + before save: again, the difference is irrelevant, so the instance will be available as instance.
• prototype.remove/destroy/delete + after save: the instance will be available as instance.

[fabien]

@bajtos @raymondfeng please review

[fabien]

This should supersede #485

[fabien]

The next step would be some 'canned' before hooks which will do the following:

Model.observe('before delete', fetchInstances); // OR:

Model.observe('before delete', fetchIds);

function fetchInstances(ctx, next) { // queryOnDelete: 'full'
  ctx.Model.find({ where: ctx.where }, function(err, instances) {
    if (err) return next(err);
    ctx.hookState.instances = instances;
    next();
  });
};

function fetchIds(ctx, next) { // queryOnDelete: 'id'
  ctx.Model.find({ where: ctx.where, fields: ['id'] }, function(err, instances) {
    if (err) return next(err);
    ctx.hookState.ids = _.pluck(instances, 'id');
    next();
  });
};

Perhaps we can even allow this syntax: Model.observe('before delete', 'presetHookFnName');

And: Model.registerObserver('presetHookFnName', function(ctx, next) { ... }).

This would be the basis for #474 (comment)

[fabien]

I'll make a new PR: #488