Calculate runtime of exited process more accurately

[amwolff]

Currently, runtime is calculated as follows:

start := time.Now()
err = process.Run()
ret.Runtime = time.Since(start)

This duration includes time for function call and so on. It is probably inaccurate.

We should use User and System Time to calculate runtime of the process (source) using os package builtin methods (link). More precisely, sum of User and Sys time.