Improve cache-handler to refetch files below 10 bytes.
Add copyright

FIXME: implement returning the latest version if ^ is prefixed to
avoid us having to import lots of old versions.

* npm-explorer.scm (read-file): Check if file is larger than 10 bytes.
* npm-explorer.scm (cache-handler): Check if empty file (= read-file
  returns #f.
* npm-explorer.scm (sanitize-version): New procedure.
* npm-explorer.scm (extract-version): New procedure.
* npm-explorer.scm (extract-deps): New procedure.
* npm-explorer.scm (get-npm-module-dot):
  Add optional parameter "version". This enables us to graph earlier
  versions to see the difference between the recursive dependency
  graphs.
  Rework the thunk to honor the versions of deps and print it.
  Change the handler to error-out if a cyclic dependency is found or a
  specific version of a package is missing.