Profiling mode

Introduce a profiling mode. In this mode each command will be timed and the results shown when the shell is exited:

```
$ phpcrsh -pjackrabbit --command="rm foo" --command="cp /cmf/contents foo" --command="save"
[0.221223] rm foo
[0.332121] cp /cmf/contents foo
[0.212312] save
```
