Improved setup.py configuration

The following fields were added:

* long_description
* license
* platforms

The following classifiers were added:

* Environment :: Web Environment
* Framework :: Flask
* Intended Audience :: Developers
* Operating System :: OS Independent
* Topic :: Communications :: File Sharing

And the ``MANIFEST.in`` file was improved.

**Still pending:** The ``LICENSE`` and ``local.py.example`` files get included in the tar.gz but they are never installed in the system when pip uncompresses the file. When using the ``data_files`` field in ``setup.py``, pip leaves the file 3 levels outside of where shiva was installed. Strange.