[boost] Moving property map library into a directory

The Boost Property Map Library (property_map) currently consists of a number of files in the top-level boost/ include directory. It would probably be better to move all of them into a subdirectory, especially as Nick Edmonds and I will be adding new files and directories into that library soon. Should I do that move? I will update Boost.Graph; are there other libraries in Boost that use property_map? -- Jeremiah Willcock