Register data Collector (issue #11)

This version introduces multiple enhancements:

added register widget (show how many objects and singletons was called)
allow to give asset url for specified data collector key

Details of additions/deletions below:
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Added:      Debug/Collectors/RegisterCollector.php
            -- Added --
                register collector basic version

Modified:   Debug/DebugBar.php
            -- Added --
                MAIN_ASSETS
                ADDITIONAL_ASSETS
                $_additionalAssetsPath contains list of additional assets
                getAdditionalPath return path for additional assets
                setAdditionalAssets set asset path for given data collector key
            -- Updated --
                __construct allow to set additional asset path

Modified:   Debug/Debugger.php
            -- Updated --
                __construct RegisterCollector usage

Modified:   examples/debugbar.php
            -- Updated --
                changed asses definition