Support for resource sets with no memory

Some tasks do not have memory assigned to them, this change allows support for this in the CLI. Those tasks cause the following exception...

```
Traceback (most recent call last):
  File "/usr/local/bin/mesos-ps", line 11, in <module>
    sys.exit(main())
  File "/usr/local/Cellar/python/2.7.6/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/mesos/cli/cli.py", line 61, in wrapper
    return fn(*args, **kwargs)
  File "/usr/local/Cellar/python/2.7.6/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/mesos/cli/cmds/ps.py", line 82, in main
    tb.add_row([fn(task) for fn in table_generator.values()])
  File "/usr/local/Cellar/python/2.7.6/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/mesos/cli/cmds/ps.py", line 44, in get_memory
    return "{0:.2f}".format((x.rss / max_mem) * 100)
ZeroDivisionError: float division by zero
```