Provide options for loading dotenv-rails earlier in the bootstrap process

https://github.com/bkeepers/dotenv/commit/da8c6a151df5654fe4a9ff8915ebaa4c87c801db moved the initialization of `dotenv-rails` from require-time to the `before_configuration` callback. This is fine for most apps, but causes issues for apps that reverence `ENV` before the `Application` constant is defined.

This PR offers 2 alternatives.

1. Manually call `Dotenv::Railtie.load` where you need it
2. `gem "dotenv-rails", :require => "dotenv/rails-now"` to load it immediately (to preserve the <1.0 behavior)

Closes #141 

/cc #10 #15 @jcwilk @JuanitoFatas
This seems could reduce the distributed gem size: https://github.com/sferik/twitter/commit/5b495e91c1496ff93def14b30f7caba1c53a5d3e