Renamed "MediaWiki" classes to use "Mw" instead for brevity
New classes for processing MediaWiki XML dumps

The main processing class is MwDumpFileFormatProcessorImpl, which reads
XML to extract revisions of pages. It forwards these revisions to a
class that implements MwRevisionProcessor. Two such classes exist:
MwRevisionProcessorBroker receives revisions and distributes them to
further MwRevisionProcessors that subscribe to specific revisions (e.g.,
only the most current revisions of type "wikidata-item"); the other
class is StatisticsMwRevisionProcessor, which gathers basic statistics
about the revisions it receives and logs them.
Test XML processing

... and some related changes (and bugfixes) in the main code.
Additional tests and better error handling
Merge pull request #36 from Wikidata/xml-dumpfile-processor

Xml dumpfile processor