Rename repo.

[sjoerdtalsma]

This repository came into existence as 'overflow' from java-globaltracer.

The code was then concerned with active spans.
During review of the initial PR it became clear the library is more about span management
and revolves around the SpanManager interface.

It would therefore be more logical to name this repo java-spanmanger or something similar.

[bhs]

Inasmuch as the "point" of this repo is a static path (perhaps via globaltracer) to an active span, there are some things I like about the current name... but agreed that java-spanmanager would be more accurate.

[sjoerdtalsma]

Ok, then I propose the rename to java-spanmanager after the initial PR #1 is merged.