Add disk queue metrics to psutil for linux

[awreece]

This enables more metrics like what would be seen in iostat -x

I investigating adding these metrics to OSX too: I suspect that kIOBlockStorageDriverStatisticsLatentReadTimeKey is what we want, but I couldn't find good documentation on it.

I ran make test on linux:

...<snip> ...
test_special_pid (test_windows.WindowsSpecificTestCase) ... skipped 'not a Windows system'
test_total_phymem (test_windows.WindowsSpecificTestCase) ... skipped 'not a Windows system'

----------------------------------------------------------------------
Ran 358 tests in 6.033s

OK (skipped=144)

And also manually verified that the new functionality is working:

>>> psutil.disk_io_counters()
sdiskio(read_count=50430, write_count=9749, read_bytes=658885632, write_bytes=98039808, read_time=117556, write_time=4380, read_merged_count=10700, write_merged_count=4712, busy_time=14836, in_flight=0, time_in_queue=71580)

[awreece]

bump?

[giampaolo]

please move this up so that only the "open_text()" call is wrapped

[giampaolo]

Sorry for taking a look at this so late. I added some comments to your PR.