Addresses issue #1330

The provider is used for debugging to help displaying current rebar's state.

Usage:
  rebar3 state

- The README is better targetted to newcomers and explains the basics of
  the project: what it is, why use it, how to get started, where to find
  more information. It looks less like a reference and more like a brief
  overview of rebar3.
- Whatever looks like a list or reference manual material is expected to
  be moved to the website
- Be clearer about the rebar3 project structure in CONTRIBUTING.md;
  explain how to write tests, how to structure code, and so on.
- Added contributing section on bug reports, feature requests, etc.

Changelog since alpha-4:

- rebar3 now bootstraps itself
- app compile fixes on app info contents and config inheritance
- dropping ErlyDTL and going to Mustache for smaller executable sizes
  and faster load time on small CPUs
- dropping `erlydtl compile` command, ErlyDTL task moved to a plugin
- default dependencies are always fetched in the right place and don't
  need to be copied across profile runs
- escriptize fixes
- `unlock` command added
- resolving/avoiding clashes when a rebar3 dep and an application
  dependency conflict together
- profile deduplication (`rebar3 as test,test,prod,test eunit` returns
  the profile/directories for `prod+test`)
- `upgrade` command bugfixes to recognize promotion of transient deps to
  top-level deps
- support template when rebar3 is run as an embedded library
- some optimizations
- parse transform support is better in erl_opts
- fixes to package handling
- fixes to `rebar3 deps`

* 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.