Windows Environment Vars are converted to all upper case

[weedySeaDragon]

addresses #291
WindowsEnvironmentVars < Environment converts all ENV keys to upper case on initialization.

WindowsPlatform now uses WindowsEnvironmentVars as its environment:

    class WindowsPlatform < UnixPlatform
      ...
      def environment_variables
        WindowsEnvironmentVars.new
      end

Rspec tests for WindowsEnvironmentVars included to be sure that accessing the environment variables by the keys will work and that WindowsEnvironmentVars.keys only has upper case chars.

• I put the class into aruba/lib/platforms but perhaps it should go somewhere else
• "WindowsEnvironmentVars.keys only has upper case chars." really means that it only checks to be sure that none of the keys match /[a-z]+/ I think that might not catch lower case unicode chars that don't get 'upper-cased' by [String].upcase
• Is this is worth including a gem that does unicode checking and 'upper-casing' so that we can be sure to check this? (like unicode_utils) Or should we wait until someone has an issue with it and then deal with it?

[maxmeyer]

Let's use if RUBY_VERSION < ' 1.9' here. And make it obvious that we need to change this code if we drop support for ruby < 1.9.3. Please no#duphere.inject` creates a new hash.

[maxmeyer]

Closed in favor of #303