Code for managing Wikimedia Foundation dump files

There are several types of dump files provided: full dumps,
dumps of current versions, and incremental (daily) dumps.
The are found online and (after being downloaded once), also
locally. The class MediaWikiDumpFile represents such a dump
and provides transparent access to its contents (whereever
it comes from, whatever type it is).

The WmfDumpFileManager provides methods for creating
MediaWikiDumpFile objects from the data seen online or in local
files. Classes that implement the interface DumpFileProcessor
can ask WmfDumpFileManager to call them for all relevant dump
files in the right order. This is the preferred way of processing
all dumps.

There are no tests yet. Testing will require suitable mock objects
to simulate the Web and the file systems. The code provides places
to inject these mock objects.

This contributes to issue #8.