Proposed B-tree library for Boost

To experiment with the library, various other boost libraries must be
available. Here is one way to create such a setup:

  git clone git://github.com/Beman/Boost-Btree.git xbtree
  svn export --force http://svn.boost.org/svn/boost/trunk xbtree
  
As a confidence test, the following should build bjam, build various
libraries and then run the Boost.btree regression tests on Windows:

  cd xbtree
  bootstrap
  cd libs\btree\test
  ..\..\..\bjam

Or on POSIX-like systems:

  cd xbtree
  ./bootstrap.sh
  cd libs/btree/test
  ../../../bjam