Allow to chain hook definitions on models

This commit allows to chain hook definitions like

Project
	.afterCreate(function (proj, fn) {/* HOOK */})
	.beforeDestroy(function (proj, fn) {/* HOOK */})
;
Merge pull request #2166 from LJ1102/chainable-hooks

Allow to chain hook definitions on models
[ci skip] Mention how to access the original sequelize methods when overriding class and instance methods. Closes #2091
Merge remote-tracking branch 'upstream/master'