* Add a provider for the xref tool for running cross reference
analysis on a project. Most of the code has been ported directly
from the rebar2 rebar_xref module with some modification and cleanup
to support testing.
* Port over the eunit test suite from rebar2, but convert it to
common_test. The testing is the same, but now the
erlang term output is examined to determine if the test run is successful
instead of scanning the console output for the expected strings.
Kelly McLaughlin