Update documentation for standalone Mac OS X CLI and grunt cli.

* When you have installed the standalone version of the xcode command line tools,
  xcode-select will fail during `npm install`, see http://stackoverflow.com/questions/12135979/node-js-can-not-install-iconv-module
* Installing grunt before grunt-cli will let the build fail. As stated on http://gruntjs.com/getting-started, `grunt` has to be
  removed before installing `grunt-cli`.
Merge pull request #89 from mfriedenhagen/mac-os-x-cli-only-documentation

Update documentation for standalone Mac OS X CLI and grunt cli.