Docs: Add XML emit to param_parse.py.  See below for more details...

I've refactored the param_parse tool to use various 'emitters'.  An emitter
can take parameter info and output it in a particular format.  Currently the
only supported emitters are the wiki and XML formats.

The goal of these changes is to create a standard machine readable description
of parameters - mainly for use by ground control stations, but it will also
enable spiffy scripting environments where code can refer symbolically to
vehicle parameters (reflectionish).

Open issue: Is there any sort of Ardupilot build id which can be included in
the generated XML?  That would ensure that we select the correct paramdefs
for the load on the target (possibly by asking the target for a SHA or
somesuch).  If that issue is resolved, then the filename for the XML
file should probably be something like:
arduplane-ca5742ac.pdef.xml.

It is worth noting that I've proposed a suffix of ".pdef.xml" for these file
types.  This facilitates automated file handling on Android devices.  On
Android you can register 'handlers' for particular file extensions and if
the user tries to open that extension in email or a web browser your app
will be given a chance to do something about it.  The 'outer' xml extension
will allow naive editors to know that at least this is an xml file.

I will include a sample of the XML format with the pull-request for this CL.