Create sane metric names when there's no pathPrefix
Allow an optional metric name prefix
Issue error message if api_token is not set in configuration

Because we can't talk to SignalFx without it.
Switch back to diamond.conf based configuration instructions

because the instructions using handler/SignalFxHandler.conf exposed a configuration loading bug
in some versions of Diamond.
Merge pull request #1 from signalfx/v0.2

v0.2