Add `Eta::Estimator`, that's independent of a Process

For times when you don't need (or have) a separate Process.

I updated the README, and added tests with minitest/spec. I also added the rake task so you can run them via `$ bundle exec rake test`.

I also made the rake dependency explicit, by adding it to the Gemfile.

I :heart: the ascii-spinner & progress bar. :smile: