Openoffice reading fixes for Nokogiri 1.5.6 changes

[] operator on XML nodes now requires the namespace to be specified
reading of ODS files was broken in roo 1.10.2 + Nokogiri 1.5.6

Signed-off-by: Karsten Richter <kasi@kasi-net.org>
generalized change for nokogiri 1.5.6 to also work in 1.5.5
new helper method "safe_attr" to safely retrieve attributes[].value
or nil if the attr doesn't exist

Signed-off-by: Karsten Richter <kasi@kasi-net.org>
Merge pull request #18 from dc7kr/master

Fixes for ODS reading with nokogiri 1.5.6