Add logging to dbt clean

[emilieschario]

This MR does two things:

1. It adds some small updates to the contributing docs to make it easier for someone to get started without getting lost in the documentation of other tools.

2. It adds logging to the dbt clean task as outlined in Feature: Command line output after dbt clean #1261.

A sample output on 0.13.0 might look like

(dbt36) >dbt clean
Running with dbt=0.13.0
Checking target/*
 Cleaned target/*
Checking dbt_modules/*
 Cleaned dbt_modules/*
Finished cleaning all paths.

[emilieschario]

@drewbanin I'd love to get your thoughts on this as is (or clarity on what else it needs).

[beckjake]

This looks good to me. Can you rebase off the existing fishtown-analytics:dev/wilt-chamberlain, or merge it in? #1378 in particular is the PR I'm interested in getting into your branch, so I can kick Windows tests off!

[beckjake]

lgtm, thank you for updating the contributing docs!

[emilieschario]

@beckjake should be good to kickoff now!

[cmcarthur]

@emilieschario I've reopened this PR with your commits against dev/0.13.1 over here: #1391. I'll merge that shortly. Thanks for contributing!!!

[emilieschario]

Thanks @cmcarthur!!