Enable verbosity config

[a-roberts]

It'd be useful to have, in appmetrics-zipkin.properties for example, and via a method call, to be able to configure how noisy the module behaves.

Either we use a standard Node.js logging library (so whatever the log4j equivalent is) or do it ourselves with custom levels - for now I'm really only looking for silent and normal.

I expect, in appmetrics-zipkin.propeties, to have:
host=...
port=...
logging=silent for example.

When the silent option is provided, i want to have zero errors reported (so when requests can't reach a Zipkin server because it's not yet configured or available for example).

With normal I'd want the behaviour as it is now. Anything extra (e.g. providing trace data) should be considered as part of #24 IMO.