Adds new command subunit-describe-calls, documentation,
and unittests.
subunit-describe-calls is a parser for subunit v1 & v2 streams to
determine what REST API calls are made inside of a test and in what
order they are called.
Later commits will add additional functionality relating to request
& response headers & body data along with a stdout output option and
stdin input.
Change-Id: I468d0d3e3b6098da95a81cc86d9bdd1b47ee1f03
c8548fc93c