bindings: add stringification shortcut for problems

Also fixes bugs in the rbsolv example.
split DEP_PRE into DEP_PRE_IN and DEP_PRE_UN

With a later commit we may use DEP_PRE_IN for uninstalled packages
and DEP_PRE_UN for installed ones.
plug mem leak in selection_make_matchdeps
add problem2str() convenience function to API
include repo_pubkey.h in repo_pubkey.c
also save/restore pool pos in repodata load callback
add changes, bump version to 0.4.0

This is done mainly so that openSUSE 13.1 will have a new minor
version of libsolv. In the future we hope to do this only for
ABI changes.
do not add back cleandeps_updatepkgs packages [bnc#841781]
add changes
fix two small glitches in xcode filter
fix unbalanced ===