tests: Make sure diff is always shown.

Even in cases, when the failure (and set -e) would end the test.
Added OSCAP_DOCUMENT_UNKNOWN to oscap_document_type_t enum, use it instead of 0
Fixed xccdf_session error message when exporting XCCDF results
Merge pull request #10 from jan-cerny/master

removed + memory leak
Create xccdf ns always in some node, don't rely on nsDef workarounds
Pass version_info to xccdf_item_to_dom

This allows tailoring export to function properly with oscap_source.
I am not terribly happy about this solution but it doesn't break
behavior and it doesn't break API. Right now the xccdf items don't have
to know their XCCDF version to function properly but that may change in
the future. At which point we will have to deal with this issue in a
cleaner way.
Merge branch 'master' of https://github.com/OpenSCAP/openscap