zipkin-test -> zipkin-tracegen

Test coverage is pretty good overall. To clarify the intent of the package it's
been moved to zipkin-tracegen.

TraceGen is set up to produce more varied service
and span names by pulling them from a pool of lorem ipsum. Trace durations have
also been randomized. This will be very handy for setting up example Zipkin
instances.

Author: @sprsquish
Fixes #350
URL: https://github.com/twitter/zipkin/pull/350