Update the netif-metrics plugin for efficiency

The `sar` call is really expensive, so avoid calling it twice, and do
all processing in Ruby
Add check-fstab-mounts plugin (Linux only)

This plugin parses `/etc/fstab` and `/proc/mounts` to ensure that all
mountpoints specified in `fstab` are currently mounted.  Optionally
filtered by filesystem type.
Merge pull request #269 from pdf/netif-metrics_perf_fix

Update the netif-metrics plugin for efficiency
Merge pull request #268 from pdf/check-fstab-mounts

Add check-fstab-mounts plugin (Linux only)
Rewrite iostat-extended-metrics plugin

Both the currently existing iostat plugins were borken in some way
(iostat-extended-metrics produced no output, and iostat-metrics
erroneously outputs headers as metrics).  Additionally neither of them
includes the `avg-cpu` stats, which can be useful for
alerting/diagnosis.

This updated plugin has more robust parsing (should not break with
banners, minor formatting changes, or added/removed fields), and
includes `avg-cpu` stats.  All included fields are dynamically generated
from output headers.
Merge pull request #270 from pdf/iostat-full-metrics

Add iostat-full-metrics plugin