Use nodetool to get VM PID

Rather than use `ps`, `grep` and `awk` to get the `beam.smp` process id,
use the `os:getpid` function.

Also, provide a `pid` command to facilitate retrieving the VM's process
id at a later time. Useful for when running via `monit`, for instance.
Add `pid` to usage
Merge pull request #284 from lukebakken/features/lrb/get-node-pid

Use nodetool to get VM PID