Updated to use WordNet 3.1 and new post-installation script

* Almost completely re-written post-installation script. The new installation download the WordNet 3.1 database directly from the [Web](http://wordnetcode.princeton.edu/wn3.1.dict.tar.gz).
* Updated `package.json` and `README.md`.
* Added `.gitignore` to prevent accidentally adding the `dict` and `node_modules` folders.
* Incremented module version to 3.1.0.

**Note** : I wrote this in a separate branch "WordNet-3.1". I suggest that, before merging, a tag called "v3.0.1" should be created to save the current version state.

```
git tag -a v3.0.1 -m 'WordNet 3.0'
git push origin v3.0.1
```

This PR might resolve issue #3 and is related to #4.