XLIFF 2 support in OmegaT

Hi everyone,

In the spirit of continuing to try out XLIFF 2.0 in different areas, I've added support for it in the filters plugin that Okapi
provides for OmegaT.

This allows the OmegaT users to edit/translate XLIFF 2 documents from their translation environment.

To try it out you'll need:
- OmegaT: http://www.omegat.org/en/downloads.html
- The plugin: http://okapi.opentag.com/snapshots/okapi-pluginForOmegaT_all-platforms_0.25-SNAPSHOT.zip

For now the filter offers only basic support. For example it doesn't honor the translate='no' annotations, or enforce
canResegment='no'. Translations in <target> is taken into account, but not the entries of the Translation Candidate module, etc. But
we'll improve this step by step in the coming weeks.

See more information on the plugin's wiki page:
http://www.opentag.com/okapi/wiki/index.php?title=Okapi_Filters_Plugin_for_OmegaT

Cheers,
-yves