require composer-installer so we can automatically copy an addOn's assets and wipe the cache on install/update

--HG--
extra : rebase_source : 426a56209e78d6844d0474c83c7a2d6260ce2ef6
fixed handling of non-existing articles in sly_Util_Article::getUrl()

Before this patch, if you use getUrl() on a non-existing ID, the getArticle()
would return the ID and not null, because the ID is also given as the default
value (to enable calls like getUrl(CURRENT_ARTICLE)).
The check for "not article" is therefore broken, as it doesn't catch $article
being a non-null integer.

The resolveConstant was added accidentally. But I left it, maybe it becomes
useful one day.

--HG--
extra : rebase_source : 91e590eb5614d7185a6a166c379ae46e6dc2be5c
change minimum mysql version to 5.0

--HG--
extra : rebase_source : f818fd466bd83e56858981c13350d94b6c5859e1