Moved the content of External to the main CMakeList.txt file.
Added documentation to the main CMakeList.txt file to make it easier to read.
Added .gitignore file to external directory to exclude source and build directories.
Change the behavior of the external Boost configure to place the Boost source, build, install directories in the respective TiledArray source, build, and install directories.
Update the TileReference and TileConstReference task_arg specializations to reflect recent changes in MADNESS.
Added a sanity check to the Boost configure script.
Changed Boost install to use the CMake install mechanism instead of a copy call.