add support for wfs:Native

[bartvde]

Add support for writing out wfs:Native in WFS transactions.

            Many times, a Web Feature Service interacts with a repository
            that may have special vendor specific capabilities.  The native
            element allows vendor specific command to be passed to the
            repository via the Web Feature Service.

[bartvde]

There appeared to be an error in the WFS 1.1 schema, preventing content to be added to the Native element, this has been recognized by the editor of the WFS spec and fixed in his version of the schema here:

 http://www.pvretano.com/schemas/wfs/1.1.0/wfs.xsd

It will go through the OGC process and be updated there as well.

[bartvde]

Working on this one now.

[bartvde]

Relevant tests pass in FF4 and Safari5, TIA for any review.

[tschaub]

Looks good. Tests pass in Chrome 12 and IE 6 as well. Please commit.

[bartvde]

(In [12123]) add support for wfs:Native, r=tschaub (closes #3349)