Moved static methods from Span to SpanHelper

Step 1 of a refactoring to give Span two implementations, one for when tracing is disabled and one for when it's enabled.
Made Span abstract and added Enabled and Disabled (currently unused) subclasses

Step 2 of a refactoring to give Span two implementations, one for when tracing is disabled and one for when it's enabled.
Changed SpanHelper to create DisabledSpan when sampling is off

Step 3 of a refactoring to give Span two implementations, one for when tracing is disabled and one for when it's enabled.
Trying to add better debug for failing remote test run.
Merge pull request #8 from yammer/null_span

Introduce Enabled and Disabled Spans