Changes

2016-06-17:
	Adding a parameter SearchParams::S which is the online
	equivalence of IndexParams::S.  At search time, all
	neighbors or a neighbor are visited at the same time
	in the original algorithm.  With the new behavior,
	neighbors are visited in batches of SearchParams::S.
	Example:
	k-NN working list:	a b c d e [f] g 
					   |
					   [l m n] o p q
	we are at entry f, and visiting it's first batch of
        neighbors l, m and n.  Suppose m and n are actually
	better than f and g, the working space will become
				a b c d e [m] n
	We'll then directly move on to check m's neighbors,
	as apposed to finish checking f's neighbors o p and q.
	Default is set to default_s = 10 which is the same for
	IndexParams::S.

2016-06-16:
        Enabling edge reversing for better search performance.
        The original index is the exact k-NN graph, and has
        difficulties to reach very high recall (>0.98) for some
        difficult datasets.  The problem is that the k-NN graph
        is a directed graph and k is the out-digree.  That means
        the in-digree of a node might have very high variance.
        If a node happen to have 0 in-dgree, then it's not
        possible to reach this node in graph-based search process.
        The idea of edge reversing is to add the reverse edges
        to the original graph for the index.  A new parameter
        IndexParams::reverse is introduced and has the following
        meaning:
                 0 :  (default) do not add reverse edges.
                -1 :  add reverse edges automatically using M values.
                k>0:  trim the graph into a k-NN graph and add all
                      the reverse edges.
        All source code using KGraph should be backward compatible,
        with a default value of IndexParams::reverse set to 0.
        For the purpose of index construction, it is recommend that
        the value be set to -1.

2016-06-02:
	Changin file format.
	Original: MAGIC, VERSION_MAJOR (2), VERSION_MINOR (0)
	Chaning to VERSION_MAJOR only, renamed as SIGNATURE_VERSION
	Using VERSION_MINOR to denote file capabiliy (SIGNATURE_CAP), which defaults to 0
	and is compatible to original scheme.