add hooks to extend the API from extension

Need
1) being able to extend the behaviour of an existing api from an extension
(eg. record an activity when the api mailing.event_bounce is called)
2) being able to replace the default behaviour of an api

Implementation (as discussed with Tim) with two new hooks:

CRM_Utils_Hook::apiRoutes($routes)
$route = array (entity,action,version)=>functiontocall)

To replace the default function called

and CRM_Utils_Hook::apiWrapper to register to wrapper

each wrapper is a class that implements fromApiInput and toApiOutput