* Distinguish between "standard" and "nonstandard" namespaces (such as vmw:).
* * When adding XML elements with specified ordering, only complain about
    unexpected elements in "standard" namespaces, avoiding noise about elements
    like vmw:CoresPerSocket
* OVF now caches self.platform to avoid repeated checks
* Cleaner handling of attributes and custom elements in OVFItem (fixes #8)
* All properties in OVFItem are now correctly converted to strings
* * This fixes some cases where we were not sharing a profile when it made
    sense to do so, so we generate slightly more optimal XML.