Add Zipkin collector support to HTrace.

* Added thrift files to talk to zipkin
* Added ZipkinSpanReceiver.that batches spans to zipkin
* Small changes to Trace
* Changed Process to be closer to Service.
* Bumped version to 2.00 since there were breaking changes.
* Make HTrace multi module
* Created htrace-core
* Created htrace-zipkin

Significant work by himanshu@cloudera.com and eclark@apache.org