Merge pull request #2: Improve platform compat w/ environment variables
Normalize setgid bits (because dpkg-deb doesn't like them)
Bug fix for unused parameter in v1.24 (see 442d67cf4dd39d71c7797c9f07540df6f715d8b4)
Make collect_related_packages() 5x faster:

 - Use high performance decorator to memoize
   overrides of Relationship.matches().

 - Exclude conflicting packages from all further
   processing as soon as they are found.

 - Removed Version.__hash__().

 - Moved the dpkg comparison cache around.
Add static analysis to detect version conflicts
Improve test coverage
Not enough entropy on Travis CI, let's see if we can work around that
Try to restore Python 3.x compatibility*

* https://travis-ci.org/xolox/python-deb-pkg-tools/jobs/33994990
Still not enough entropy on Travis CI, let's see if we can work around that

I tried to fix this using rng-tools in 3c372c3097f77fa4f6d8ff7c67a8e18
but that didn't work out due to the way OpenVZ works. This commit
introduces a more general approach that will hopefully work
on OpenVZ and other virtualized environments, we'll see...
Try to restore Python 3.x compatibility*

* https://travis-ci.org/xolox/python-deb-pkg-tools/jobs/33995580
I wonder how far I'll have to go with this ..
Nothing to see here, move along ...
Cleanup .travis.yml after experimentation
Make collect_related_packages() a bit faster
Command line interface for static checks (with improved test coverage)
Bug fix for SQLite cache string encoding/decoding on Python 3.x
Bug fix for check_package() on Python 3.x
Make collect_related_packages() a bit faster*

* Actually quite a lot when `dpkg --compare-versions' is being used :-)
Make deb_pkg_tools.control.* less verbose
Bug fix for obscure Python 3.x issue*

* Caused by mutating a list while iterating it.