Language::Node.npm_install_args: add helper method (#37)

* add Language::Node helper module

This adds a language module for Node module based formulas.
It contains the 2 public methods `std_npm_install_args(libexec)` and
`local_npm_install_args`:

* `std_npm_install_args` is intended to be used in formulas for
  standard node modules and returns `npm install` args for a global
  style module installation to libexec.
* `local_npm_install_args` is for formulas, in which the `npm install`
  step is only one of multiple parts of the installation process and
  returns `npm install` args for a default local installation in place.

Both methods have in common, that they are

* making sure that a working copy of npm and node-gyp from node's
  libexec is prepended to the PATH (to not rely of a user managed npm)
* seting the npm cache to HOMEBREW_CACHE/npm, which fixes issues caused
  by overriding $HOME resulting in long install times + high disk usage
  (see https://github.com/Homebrew/brew/pull/37#issuecomment-208840366)

* audit: update npm install check for Language::Node

* cleanup: remove npm_cache too

* doc: add Node-for-Formula-Authors.md

1 files changed, 7 insertions, 0 deletions

diff --git a/Library/Homebrew/test/test_cleanup.rb b/Library/Homebrew/test/test_cleanup.rb
index bccfa1e54..1351c85a6 100644
--- a/Library/Homebrew/test/test_cleanup.rb
+++ b/Library/Homebrew/test/test_cleanup.rb
@@ -73,4 +73,11 @@ class CleanupTests < Homebrew::TestCase
     shutup { Homebrew::Cleanup.cleanup_cache }
     refute_predicate java_cache, :exist?
   end
+
+  def test_cleanup_cache_npm_cache
+    npm_cache = (HOMEBREW_CACHE/"npm_cache")
+    npm_cache.mkpath
+    shutup { Homebrew::Cleanup.cleanup_cache }
+    refute_predicate npm_cache, :exist?
+  end
 end