To add a new project to third_party, do the following:

1) GIT: Run
      git clone git@github.com:Khan/<project>.git <project>-src
   'git clone http://...' should work too, but uploading will be more work.

   MERCURIAL: Run
      hg clone https://.../<project> <project>-src

   Note that in either case, we give the project a -src suffix!  Also,
   we should only import projects that are under a user we own (such
   as the Khan group on github).  Use the forking feature of the
   project-hosting site to make a fork of the project if necessary.

2) Run
      ln -s <project>-src/<something> <project>
      hg add <project>
   <something> should point to the root of where the actual python
   source code is.  We need this because we do not install the python
   project normally, via 'pip install' or the like.  Instead, we use
   this symlink to point to the root of the library code.

   If the project does non-trivial work at install time (such as
   building .so files, etc), we will need to augment the steps here.

3) Update ../.hgsub with the information for this repository, a line like
      third_party/<project>-src = [git]git@github.com/Khan/<project>
      third_party/<project>-src = [hg]https://.../<project>

4) Now you can 'hg commit' the .hgsub file and your symlink from step
(2), and you should be done!

Note: I think the default is that you get the version that you had
checked out when you added it, unless you specify otherwise.
TODO(csilvers): how would you specify otherwise?  hg checkout?