Rewritten update script and configuration

LU rewrite part 5: The update.php was rewritten to be a maintenance
class and use the new helper classes. New configuration format for
repositories, which is not backwards compatible. Will use GitHub
as default as it provides the list of files in a directory,
needed for efficient fetching of json translations.

Cleaned up LocalisationUpdate class of now unused code.

Speed seems reasonable. With translatewiki.net like setup, speeds
were as following:

Local clones:
Saving 2866 new translations

real    1m26.045s
user    1m25.149s
sys     0m0.688s

From GitHub
Saving 2866 new translations

real    5m4.899s
user    1m38.182s
sys     0m1.788s

Updates could be further optimized by not fetching files which have not
changed a) since last update, b) from what is currently in use. Fix a)
would require to store what translations were updated for each file.

This extension fully supports json.
1) Extensions using json files are supported, picking up new languages
as they appear.
2) Core is easy to switch to json once it has been converted.
3) LocalisationUpdate stores updated translations in json format.

There are a few caveats, though.
4) Once core switches to json format, all users must update LU to keep
receiving updates. But they will need to update anyway because older
versions of LocalisationUpdate do not support json at all.
5) If an extension switches to json, we will not be able to provide
updates until its local checkout is updated to such a version:
LocalisationUpdate does not know about the switch and will try to fetch
the PHP shim, which does not have translations any longer. This is
usually okay, as the PHP shims support old MediaWiki versions; problems
might appear if the extension has other incompatible changes which
prevent updating.

I have tried to rewrite LocalisationUpdate in a way compatible with
MediaWiki 1.19, including being compatible with PHP 5.2.3. After
support for that version is dropped, some code can be improved to take
advantage of new features in PHP 5.3, mainly namespaces.

Change-Id: I21ce94ae2262d9c90132d21c4c9f8ebdd08a14ea