Addressing issue #131 -- Add I/O metrics on Mac

Apple's version of iostat reports different metrics than Linux, FreeBSD, or SunOS. This adds
three new metrics under system.io to log sectors per second (system.io.sectors), transfers per
second (system.io.transfers), and milliseconds per seek (ms_per_seek).
Add unit test for _parse_darwin

Checks that sample output from iostat on OS X 10.8 parses correctly
Rename 'ms_per_seek' to 'ms_per_transaction'

The manpage for iostat on Mac uses both terms, but
Renamed Darwin disk metrics, discarded msps

system.io.sectors is now system.io.sectors_per_s,
system.io.transfers is now system.io.transfers_per_s,
and system.io.ms_per_transaction is removed because it is
inconsistently defined in the manpage (and always reads 0.0)
Merge pull request #530 from DataDog/issue131

Addressing issue #131 -- Add I/O metrics on Mac