Bug fix to create cached directory if it doesn't exist

Fixed a bug where the tar command would fail on some systems if the cached directory did not exist.

Added command-line options + updated README

Added command-line options for npm-cache:

Examples:
  npm-cache install                                     try to install npm, bower, and composer components
  npm-cache install bower                               install only bower components
  npm-cache install bower npm                           install bower and npm components
  npm-cache install --cachedDirectory /Users/cached/    try to install npm, bower, and composer components, using /Users/cached/ as cached directory

npm-cache executable from command line

added "bin" and "preferGlobal" fields to package.json so npm-cache can be run from command line

Firs release of npm-cache

npm-cache gives you the ability to cache installs from package managers including npm, bower, and composer (for now!)