Added new features for virtual/swap memory stats.

Added ps.virtual_memory and ps.swap_memory to replace the following
deprecated functions:

 - psutil.phymem_usage()
 - psutil.virtmem_usage()
 - psutil.cached_phymem()
 - psutil.phymem_buffers()
 - psutil.avail_phymem()
 - psutil.used_phymem()
 - psutil.total_virtmem()
 - psutil.avail_virtmem()
 - psutil.used_virtmem()
Update get_boot_time function.

Change psutil.BOOT_TIME to psutil.get_boot_time() as the
former is deprecated.  Output remains unchanged.
Updated network io counter function.

Change psutil.network_io_counters() to psutil.net_io_counters()
as the former is deprecated.  Output remains unchanged.