Upgrade psutil dependency to version 2.1.1

That was far too simple...
Improved various metrics - more useful memory metrics, fds/io, system uptime

New metrics:
	supermann.metrics.system.load_scaled
	supermann.metrics.system.uptime
	supermann.metrics.process.io

Modified metrics:
	supermann.metrics.process.mem
	supermann.metrics.process.fds
	supermann.metrics.system.mem

Closes #9