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
Merge pull request #6 from elliottneilclark/zipkin

Add Zipkin collector support to HTrace.
Clean up maven deprication warnings.
Merge pull request #7 from elliottneilclark/maven_deprication

Clean up maven deprication warnings.
Move the zipkin thrift file into git

This should allow developers to build htrace without needing thrift and
also allows us to remove a dependency in the build.
Merge pull request #8 from elliottneilclark/remove_thrift_gen

Move the zipkin thrift file into git
Fix gpg sign on deploy
Merge pull request #9 from elliottneilclark/fix_gpg_sign

Fix gpg sign on deploy
[maven-release-plugin] prepare release htrace-2.00
[maven-release-plugin] prepare for next development iteration
Remove synchronized on Tracer#getInstance

Move to the initialization on demand holder idiom to remove lock
contention after first initialization.
Make sure that no CME is possible in TraceTree.

HBase was seeing CME's sometimes in tests as the collections of spans
being created were being modified.  Create a copy so this isn't
possible.
Merge pull request #12 from elliottneilclark/cme_tracetree

Make sure that no CME is possible in TraceTree.
Merge pull request #11 from elliottneilclark/tracer_no_sync

Remove synchronized on Tracer#getInstance
[maven-release-plugin] prepare release htrace-2.01
[maven-release-plugin] prepare for next development iteration
Detached spans were never sending.
Merge pull request #13 from elliottneilclark/close_spans

Detached spans were never sending.
[maven-release-plugin] prepare release htrace-2.02
[maven-release-plugin] prepare for next development iteration