Adds an environment_vars_fallback to the Settingslogic class

I wrote this to play nicer with Heroku or Travis-ci, where it's easier to set environment variables. 

If set to true, non existing settings attempt to retrieve value from
environment variables. For example:

```ruby
class Settings < Settingslogic
   source "#{File.dirname(__FILE__)}/settings.yml"
   suppress_errors true
   environment_vars_fallback true
 end

ENV['SETTINGS5_NON_EXISTENT_KEY']='foo'
Settings5.non_existent_key.should == 'foo'
```