Tweaks for SiteConfiguration:

Tweaks for SiteConfiguration:
* Introduced SiteConfiguration::$siteParamsCallback, a callback function that returns an array of params to define custom parameters to replace and wiki tags so that they can be used at run time (e.g. for CentralAuth or SiteMatrix), can also override SiteConfiguration::siteFromDB(). See the documentation of that variable for further information. Backward compatibility is maintened for paramters passed to SiteConfiguration::get() and similar functions but the vaules returned by the callback function will override them.
* Added SiteConfiguration::arrayMerge() for merging arrays, written by Werdna. Warning: as array_merge(), it may change numeric keys, so don't use it for settings having namespaces index in keys.
* Removed SiteFromDB hook, deprecated by the callback function
* document a bit