Goal: excluding and then installing results in incomprehenisble problem desc. (RhBug:995459)

Patch concept by Michael Schroeder <mls@suse.d>. Thank you!

There still are alternative approaches to excludes: dropping packages them from
the repos or by using pool->considered, yet so far nothing indicates we need to
go there.
New version: 0.4.2
build: 0.4.2-1
tests: move TestSack out of the testing module into tests.

Tailored load_yum_repo() is moved into the TestSack and TestSack is moved out of
hawkey.test: clients like DNF do not care for hawkey-tailored
load_yum_repo(). Better leave them access to the real load_yum_repo and let them
override.