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

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