Allow environment variables in the configuration file

[IamNaN]

This eases deployment by allowing environment variables in private_pub.yml.

Because Faye needs to run in production mode, the yaml file's production block gets used for both development and production, making private_pub hard to deploy.

production:
  server: <%= ENV["FAYE_SERVER"] %>
  secret_token: <%= ENV["FAYE_KEY"] %>

This change also forces signature_expiration to an integer regardless of how it is entered in the configuration, and it adds tests.

[IamNaN]

Also, I required yaml in private_pub.rb because tests weren't passing without it, even before this pull request.

[rubytastic]

This would have been interesting, How about Puma? what is needed to get puma to work with private_pub,

Changing adapter to puma still gives a

19:39:24 privpub.1 | /Users/Jordan/.rvm/gems/ruby-2.0.0-head@books/gems/rack-1.5.2/lib/rack/handler/thin.rb:1:in `require': cannot load such file -- thin (LoadError)

Where/ How to overrule this?

[gakshay]

@ryanb How about merging this pull request. Having a similar need to use the Environment variables on production..

[ferydjzz]

+1 I also need to use ENV['variable'] on production