Added in running stats for btt

Added in running stats for btt

Create an overall system and per-device statistics file containing
MB-per-second and I/Os-per-second values. Format for each file is first
column contains an (integer) time stamp (seconds since start of run)
and a (double) value.

File names are:

sys_mbps_fp.dat - system-wide mbps (for all devices watched, of course)
sys_iops_fp.dat - I/Os per sec

Each device watched will have a file with the device preceeding the
_mbps or _iops section of the above file names.