[boost] (new) Boost.Plugin library

Hi, I have written a plugin registration library from scratch during the past few weeks. There seems to be a Boost.Plugin library (that I was not aware of) out there, but it isn't part of the distribution, so it seems fair to reuse the name. My implementation is far more generic than the old one, and not really aimed for dynamic libraries (that's just something that comes for free). A draft (for comments) is here: http://zi.fi/plugin/