* 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.

Removed separate compilers
Resolves apps to build
Finds avail deps before pulling/building
Includes relx
Simplifies build commands

While at it, refactor the guide for clarity. Some of the new text was
taken from erlware/relx/CONTRIBUTING.md and modified as needed.

Links to the mailing list, issues page, and wiki are included,
plus information relative to the IRC channel.