* 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.

support for provider_hooks in config

no more mode minimal

app_info:valid needs to return a boolean and not throw an exception

use processed app file for validation

fix typo in error tuple and throw error in install_deps

Clean up and backwards compatability

check locks before locking a dep to not have duplicates

add emulator args to escript for faster load time

don't lose erl_opts when compiling for tests

Error dep parsing

generalize some temporary directory handling and add it to `rebar_file_utils`

swap `file:list_dir` for `file:list_dir_all` due to nonexistence of latter prior to r16x

do provider returns the error of the first provider to fail and stops

no skip warning for seen dep that has locked version

(in `rebar_file_utils`)

change more instances of mistyped `erl_first_modules` to `erl_first_files`

`erl_first_files`

fixes #99

don't mark dep as seen before calling handle_dep

replace single profile atom in providers with list of profiles

when compiling `common_test` or `eunit` use the same tmp dir

from run to run

change `rebar_app_into` to `rebar_app_info` in typespecs

test providers