Clarify licensing situation of VertexQueue.

  * VertexQueue was derived from Bearophile's RosettaCode
    example of a GrowableCircularQueue.  The default licensing
    of RosettaCode (GNU FDL) would have precluded its use
    in a GPL-licensed work.

  * Bearophile was kind enough to offer the code under the Boost
    license as a resolution to this licensing quandry:
    http://forum.dlang.org/post/mvmzvkjpbhazezlsydim@forum.dlang.org