ODF validator integration

[sander]

Integrate one or more ODF validators in Officeshots. Both requests as results should be run through a validator to check the documents that are being uploaded.

The results of the validation should be visible to the user. Caveat: It's no use to spam regular users with XML Schema errors. The validation results should be human readable. The detailed validation results should be accessible as well for application developers working on interoperability.

[sander]

Alex Brown has written an online validator that can be of use:
 http://www.probatron.org:8080/officeotron/officeotron.html

It is also usable as a webservice. You an simply POST a document to:
 http://www.probatron.org:8080/officeotron/validator

[sander]

The people at Cyclone3 CMS have also written a validator:
 http://www.cyclone3.org/documentation/addons/extensions/ODFvalidator/

They have offered to rewrite it as a stand-alone script instead of a CMS plugin, which we could use on our own server.

[sander]

Additionally there now is an ODF-diff tool from DeltaXML. It could compare ODF roundtrip documents with the original.

[sander]

Fixed with r285. Implemented validators:

* Cyclone3
* Office-o-tron
* ODFToolkit