Per-method hooks

[bajtos]

A follow-up for #367.

As a LoopBack user, I want to execute my custom piece of code before/after a particular method is executed.

• the handler is async
• the handler can change input arguments and/or result (arguments passed to the callback)
• both static and prototype (instance) method can be overriden
• handlers are inherited by subclassed models

@raymondfeng suggested using a ctx object instead of a list of arguments in the handler