Updated changelog

ChangeLog

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+Version 1.6.8
+	* bug fixes, low dimensional search speedup
+
+Version 1.6.7
+	* bug fixes
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+Version 1.6.6
+	* misc bug fixes
+
+Version 1.6.5
+	* fix compilation problem on some C++ compilers
+	* fixes in the python bindings
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+Version 1.6.4
+	* small bug fix
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+Version 1.6.3
+	* radius search speedup
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+Version 1.6.2
+	* slight API changes to the C++ bindings, now the main index type is templated
+	* on the distance functor which makes it easier to use custom distances
+	* new kd-tree implementation optimized for low dimensionality data
+	* experimental MPI support for cluster computing
+
 Version 1.5
 	* new C++ templated API
 	* saving/loading of indices to disk