* split Meta.h to Meta.h (generic meta programming) and XprHelper.h (relates to eigen mechanism)

* added a meta.cpp unit test
* EIGEN_TUNE_FOR_L2_CACHE_SIZE now represents L2 block size in Bytes (whence the ei_meta_sqrt...)
* added a CustomizeEigen.dox page
* added a TOC to QuickStartGuide.dox

Eigen/CoreDeclarations

@@ -47,6 +47,7 @@ namespace Eigen {
 #include "src/Core/util/Constants.h"
 #include "src/Core/util/ForwardDeclarations.h"
 #include "src/Core/util/Meta.h"
+#include "src/Core/util/XprHelper.h"
 #include "src/Core/util/StaticAssert.h"
 
 } // namespace Eigen