Add psutil dependency

[m2-farzan]

Hi, thanks for this fantastic project.

The ./run script fails to start inside a venv with the following error:

Traceback (most recent call last):
  File "/home/mostafa/tmp/rosboard/./run", line 3, in <module>
    import rosboard.rosboard
  File "/home/mostafa/tmp/rosboard/rosboard/rosboard.py", line 23, in <module>
    from rosboard.subscribers.processes_subscriber import ProcessesSubscriber
  File "/home/mostafa/tmp/rosboard/rosboard/subscribers/processes_subscriber.py", line 3, in <module>
    import psutil
ModuleNotFoundError: No module named 'psutil'

It can be fixed by installing psutil. So I've added it as a dependency.

[m2-farzan]

Oops... psutil seems to be required only for system_stats_subscriber feature. I'll make it optional. There's also an extra import that should be removed.

[dheera]

Thanks!