patch to match latest version of Mozilla code

Not an exact copy of https://developer.mozilla.org/en-US/docs/JXON#Appendix.3A_a_complete.2C_bidirectional.2C_JXON_library;
I undid some formatting changes to get `git diff` to only display the relevant
changes - namely, adding the `sNamespaceURI` and `sQualifiedName` parameters to
`JXON.unbuild`.

The Mozilla code also doesn't properly propagate element namespaces in
`loadObjTree` or handle non-null namespaces in `JXON.unbuild`; these bugs are
fixed in my version.
let an element specify a custom namespace with the "@xmlns" property
bring back `JXON.stringify`

Accidentally deleted it in commit a42ae3d.  This brings it back and adds the
parameters `sNamespaceURI`, `sQualifiedName` and `oDocumentType` (the same
as in `JXON.unbuild`.)