Move autoloading to dotenv-rails gem

I should have heeded the advice in #14 and created a different means besides `require "dotenv"` to kick off the autoloading. This attempts to rectify it by moving the autoload behavior at least to a different gem.

My thought is to release this as 0.6 which gives the following deprecation warning:

```
[DEPRECATION] Autoloading for dotenv has been moved to the `dotenv-rails` gem. Change your Gemfile to:
  gem 'dotenv-rails', :groups => [:development, :test]
```

I'll then follow up with a 0.7 release that removes the autoload behavior completely from `lib/dotenv.rb`

Thoughts?