Script for local data download

This pull request includes a bash script (tools/download.sh) that installs nltk_data collections from a local copy of the repository.

This provides a viable workaround if the nltk.download() tool does not work. (e.g. there are reports of this tool hanging on stack exchange and in the issue log: https://github.com/nltk/nltk_data/issues/7) that were also encountered by this author. 

The script uses python for parsing the xml and the standard tools: find, basename, dirname, readlink, mkdir, cp pwd and unzip

The script is currently limited to installing entire collections, not individual packages