added SLY_SLICE_PRE_RENDER and SLY_SLICE_POST_RENDER events

--HG--
branch : 0.8
fixing bc compatibility sly_Response when setting headers in constructor

--HG--
branch : 0.8
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--
branch : 0.8
change minimum mysql version to 5.0

--HG--
branch : 0.8
the event listener should get the article of the expected lang

--HG--
branch : 0.8
lessphp custom function to add timestamp to asset urls

--HG--
branch : 0.8
added medium->getUrl()

--HG--
branch : 0.8
fixed sly_Layout::getBodyAttr

--HG--
branch : 0.8
added parameter to start session only if cookie is set, added methode kill session and session cookie

--HG--
branch : 0.8
merge with 0.8