process: add process.cpuUsage() - implementation, doc, tests

Add process.cpuUsage() method that returns the user and system
CPU time usage of the current process

PR-URL: https://github.com/nodejs/node/pull/6157


Reviewed-By: Robert Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>