Send timers as gauges instead of absolute gauges.

The documentation on the difference between the two is minimal but it
seems that an absolute gauge will always overwrite the value with the
last one received. Based on this it would seem to be better to send a
gauge as Instrumental should store an average or combination of the
values.
Add a note to the readme about `flush_interval`.

We noticed odd issues with our production metrics and realized that
Instrumental was either averaging or overwriting metrics that were
flushed more often than the highest supported resolution (60s). To solve
this we increased the flush interval to allow the metrics to be
aggregated by statsite. This greatly improved the accuracy of our
metrics and should be noted for anyone using our sink.