use the go agent to generate traces

[graphaelli]

closes elastic/hey-apm#90

Note that this disables interactive mode for now.

Some ideas for follow up:

• figure out how to get span count from the agent
• use a custom transport for tracking HTTP stats - response codes, latency, bytes sent, etc
• generate errors
• add workload profiles - transaction and error mixes that would be difficult/impossible to specify on the command line