Add the Commons as a supported language

Most of the extraction framework works just fine for the Commons if it is treated as a separate language. This pull request includes a extraction.commons.properties file that demonstrates this, and modifies the extractors so that they parse articles in the 'File' namespace if the current language is "commons". It allows basic RDF to be extracted from Commons dumps.

This is my first pull request as part of my Google Summer of Code project, so it should not be merged until @jimkont has had a chance to look it over and give me feedback!