Improve packaging - Round 2

* markdown -> reStructuredText: Python services (like PyPI) only support rst. So, README.md -> README.rst

* split out the user guide from README into new ``glances-doc.html``.
The source file is written in rst as well. To generate the HTML file, you need the [docutils](http://docutils.sourceforge.net/) package, then run:

```console
rst2html --stylesheet=stylesheet.css glances-doc.rst > glances-doc.html
```

* add classifiers info to setup.py for a better PyPI integration
* update man page with some improvements
* better layout packaging and consistency