Allow Ettu to be configured

Allows Ettu to be disabled using the standard rails config optoins, and
adds in a flag to workaround
[#10791](https://github.com/rails/rails/pull/10791).
Move Ettu class into the top level.

No more Ettu::Ettu.
Update README

Whitespace and Rails config options.
Allow configuration of defaults
Add #asset_etags and make #asset_etag private
Rewrite README to include .configure
Bump version to 0.0.4