Fix handling of errors raised when undoing transactions.

Fixed message logged when undoing an unavailable package and
AssertionError raised when undoing a not installed package.
Fix AssertionError raised when undoing a package available in multiple repos (RhBug:1038403)
Let install command recognize '@' (RhBug:1036211)
Let history info command recognize 'last'.
Fix TypeError raised when comparing YumHistoryPackage in Python 3.
api: plugins: add hook for sack ready (RhBug:1038937)
tests: add repository with delta rpms
tests: test drpm lookup, test basic rpm downloading