Don't store the owner document instance in each nodes.

[angelozerr]

Don't store the owner document instance in each nodes (see #389). This PR improves the memory with big file (there is around 1GB gain with nasa.xml file)

Signed-off-by: azerr [email protected]

[angelozerr]

Here the result when nasa.xml is parsed with DOM Parser with existing code:

Here the result when nasa.xml is parsed with DOM Parser with this PR (owner document is not stored in each nodes instances):

There is around 1GB of gain.