Improved handling of the package version number

- Stored in a single place: module activepapers.version
- Made accessible through aptool --version
Import HTTPError to activepapers.url
Add support for automatic download from Zenodo.org
Store external dependencies as vlen strings

This is mostly for consistency, since most other string data
is stored as vlen strings as well. It also helps with Py2/Py3
compatibility.
Fix two small resource leakages
Set version number to 0.1.1