Sitelink support (merge only after #55)

Several extension to add sitelinks support (issue #37). This is based on the changes done in #55, which should be merged first.

* Extend dump file management to support gzipped dump files that do not
contain page revisions
* Simplify some methods in the file/Web utils for better support of
various compressions
* Simplify and generalize some methods in the dump file management;
fewer method names refer to specific dump files now
* Support new dump type "SITES" for sites table dump
* Add Sites interface and implementation to datamodel to hold and use
sites data
* Add new dump file processor for parsing SITES dumps to get Sites
object
* Add example program that shows how instrument all this to make use of
sites data
* Renamed MwDumpFileProcessorImpl to MwRevisionDumpFileProcessor, since
we have several such classes for several types of dumps now
* Updated processing code and tests to work with the changes