Make "lib/node" a real folder.

Treat the "lib/node" folder like Python, Perl, etc.
Create it as a real folder in HOMEBREW_PREFIX, so that
package managers (npm) have a stable place to drop libs.

Library/Homebrew/keg.rb

@@ -57,6 +57,7 @@ class Keg <Pathname
       # lib/language folders also get explicitly created
       when 'ghc' then :mkpath
       when 'lua' then :mkpath
+      when 'node' then :mkpath
       when 'ocaml' then :mkpath
       when /^perl5/ then :mkpath
       when 'php' then :mkpath