v0.12.0 - Single error callback for network based reporters

#27 - To improve memory utilization, reporters will stop getting the callback upon invocation of a report, but rather get an error callback upon creation.

BREAKING CHANGES

• Reporters - to get errors from a reporter, initialize it with its own error callback in addition to the error callback in the main Metrics object
• Reporters - error callback will only be triggered upon error, and not for each invocation
• No default callback (was previously console.error) for errors.