Add Elemental subtree

 I have decided that git-submodules is probably not the right solution for us. The main disadvantage I found (after trying to write the documentation) is that it changes the workflow of everyone, requiring users to add extra flags or run additional commands. If someone forgets these extra flags, it has the potential to screw things up for everyone. 

Instead, I propose we use git-subtree. The advantages of this approach are:
- a simple clone will pull Elemental with MADNESS (no extra commands/flags require)
- we don’t have to modify makefiles or configure scripts (e.g. uncompress tarballs or update submodules)
- users do not need to learn or do anything new w.r.t. git commands
- subtree does not add new metadata files like submodules doe (i.e. .gitmodule)
- we can modify elemental files without maintaining a fork of Elemental
- it works with older version of git

Updating Elemental with git-subtree is relatively simple.
$ git subtree pull  --prefix=src/elemental --squash https://github.com/elemental/Elemental.git <ref>
$ git commit -a -m "Updated Elemental to <ref>"
$ git push
where <ref> is a branch, tag, or SHA. User only need to "git pull" to get the changes.

If we want to modify files in elemental, then we can do so as if they were any other file in MADNESS.