Enable local spans

[DanielMSchmidt]

There is a type of span in zipkin that does not resemble a client request or a server request. We should therefore introduce the kind "local" to do this. The current implementation is documented here: openzipkin/zipkin-js#104

[DanielMSchmidt]

Waiting for openzipkin/zipkin-js#156 to be merged

[DanielMSchmidt]

The example needs to be rewritten to account for local spans