Bug#733489: python-apt: Improve 'Dependency' and 'BaseDependency' to get target package versions that satisfy dependencies

[Michael Schaller <michael@5challer.de>]

Before I start with the BaseDependency properties here two more proposals:

apt/package.py: Improved readability and documentation of 
BaseDependency.__dstr().
https://github.com/michael-schaller/python-apt/commit/ef6202e2f54dc2085e4d87a01aa2a66ff7c1a437

apt/package.py: Simplified BaseDependency constructor.
https://github.com/michael-schaller/python-apt/commit/928814bbc43ed50ab956271a514baee49c7a452b

Julian, how do you like these two patches?