Initial Play tracing SPI

Included is a proposed tracing SPI for Play.  It is a reification of the
Play support for console does (with a number of fixes, improvements and
simplifications).  This work is on-going but is now in a form that is
ready for some initial review.

The primary tracing callback API is represented as Java interfaces and
can be found in the `play-instrumentation-spi` sub-project.

There is an example implementation of the SPI that computes average
statistics for requests.