Remove the need for threads for meters

thanks to the wonderful work in codahale/metrics@c72830f27
Release 0.9.9.2
stddev is the square-root of the variance
Close the tcp socket on Errno::EPIPE.
Add trouble shooting link to README

Make the troubleshooting page more prominent so users do not chase down weird behavior in their production environments.
Merge pull request #31 from xrl/patch-1

Add trouble shooting link to README
Merge pull request #29 from jordansissel/patch-1

stddev is the square-root of the variance
Release 0.9.9.3
Merge pull request #30 from bernd/tcpsocket-epipe-fix

Close the tcp socket on Errno::EPIPE
Release 0.9.9.4
Add a Gauge class.
Add gauge to the registry and create a Metric.gauge method.
Add gauge to reporters.
Add gauges to the README.
Fix typos in comments.
Merge pull request #34 from bernd/typos

Fix typos in comments.
Add a #callback method to the gauge class.
Remove callback method and pass the callback to the constructor.

This looks way nicer and matches how it works in Coda Hale's metrics.
Do not use a default callback and just call it if there is one.
Use a gauge default value of nil.