Be more specific, look for IODEF-Document element with namespace

Previously the IODEF parser would look only for the existence of the
IODEF namespace in the element tree. Now we explicitly look for the
correct element name (IODEF-Document) and the correct namespace
within that element.
Cosmetic - Rename item to iodefXml for readability
Support parsing IODEF whether root element or nested

Whether the IODEF to parse is wrapped in other elements (e.g. <item>) or
it is the root element the parser should still handle it.
Added a test for parsing IODEF wrapped in another element
Merge pull request #7 from surevine/better-iodef-detection

Better iodef detection