Expose public singleton instance of NoopTracer (#39)

Author: yurishkuro

opentracing-impl/src/main/java/io/opentracing/NoopTracer.java

@@ -17,6 +17,8 @@
 
 public final class NoopTracer implements Tracer {
 
+    public final static NoopTracer INSTANCE = new NoopTracer();
+
     @Override
     public SpanBuilder buildSpan(String operationName) {
         return NoopSpanBuilder.INSTANCE;