[maven-release-plugin] copy for tag 0.15.0 

Merge pull request opentracing#51 from objectiser/spanfields Make the fields in the abstract span available to concrete impls

[maven-release-plugin] copy for tag 0.14.0 

Merge pull request opentracing#40 from opentracing/bhs/mocktracer Add a MockTracer to aid with unittests

[maven-release-plugin] copy for tag 0.13.0 

Expose public singleton instance of NoopTracer (opentracing#39) 

[maven-release-plugin] copy for tag 0.12.0 

Merge pull request opentracing#35 from opentracing/bhs/explicit_finish Support explicit Span finish times

[maven-release-plugin] copy for tag 0.11.0 

Merge pull request opentracing#34 from opentracing/bhs/iterable_textmap Support Iterable via a method rename