Support Sharable Span and Http Collector

This release contains two important changes:
a) Tracer now supports sharable(single) and non-sharable(dual) styled spans. Sharable span is where client and server span share the same TraceId, SpanId and ParentSpanId. Non-Sharable(dual) style generates server span as a child of client span. The default is sharable, but one can enable tracer to work in dual span mode.

b) Added support for http dispatcher. Besides, haystack-agent(grpc sidecar) as a dispatcher, now we also allow spans to be dispatched to a http collector. For more information read here

Bug Fixes

This release contain a bug fix where we set ParentSpanId as null (if not set) instead of initialising as UUID.zero.

Upgrade haystack-commons to remove logback.xml file

0.1.4

Merge pull request #56 from ExpediaDotCom/BumpHaystackCommonsVersionT…

Logging Improvements

Merge pull request #54 from ExpediaDotCom/release-0.1.2

releasing 0.1.2