Add `Dotenv.env` and `Rails.configuration.env`

This exposes a wrapper around ENV that allows you to AVOID_SCREAMING_SNAKE_CASE_ALL_OVER_THE_PLACE. This implementation feels like a sane balance between just using `ENV` and the bloated configuration frameworks.

From the updated readme:

## Usage

…

Whenever your application loads, these variables will be available in `ENV` and a `Doten.env` helper object:

```ruby
s3 = AWS::S3.new({
  :access_key_id     => ENV['S3_ACCESS_KEY'],
  :secret_access_key => ENV['S3_SECRET_ACCESS_KEY']
})

# or

s3 = AWS::S3.new({
  :access_key_id     => Dotenv.env.s3_access_key,
  :secret_access_key => Dotenv.env.s3_secret_access_key
})
```

### Rails

In Rails, `#env` is available on the `Rails.configuration` object, in controllers…

```ruby
class ApplicationController < ActionController::Base
  before_filter :redirect_unknown_hosts

  def redirect_unknown_hosts
    if request.host != env.app_host
      redirect_to "#{env.app_host}#{request.path}"
    end
  end
end
```

…and in views.

```erb
<!-- app/views/layouts/application.html.erb -->
<script>
  // ...
  _gaq.push(['_setAccount', '<%= env.google_analytics_id %>']);
  _gaq.push(['_setDomainName', '.<%= env.app_host %>']);
  // ...
</script>
```

## Customization

Logic-less configuration is ideal, but unfortunately we don't live in an ideal world. You can add logic to your configuration by extending it with your own module:

```
# lib/my_app/config.rb
module MyApp
  module Config
    def redis_url
      self['REDISTOGO_URL'] || self['GH_REDIS_URL'] || "redis://localhost:6379"
    end
  end
end

# config/application.rb
module MyApp
  class Application < Rails::Application
    # ...

    require 'my_app/config'
    config.env.extend MyApp::Config
  end
end

# config/initializers/resque.rb
Resque.redis = Redis.connect :url => Rails.configuration.env.redis_url
```