Pull C++ sources from external repository

Hi,
after a discussion with @mreineck, we decided better avoid keeping a clone of the `HEALPix` C++ sources inside `healpy`.

Therefore I removed the `hpbeta/` folder.

I am maintaining a git read-only mirror of the HEALPix SVN repository on http://github.com/healpy/healpix

This way, we can include it in `healpy` using `git submodule`, see http://git-scm.com/book/en/Git-Tools-Submodules

therefore now the developers need to checkout healpy, then run:

    git submodule init
    git submodule update

in order to pull the correct version of HEALPix from the repository.
Then they can build with the usual:

    python setup.py build

Please test this checking out the `rebwithlibsharp` branch:

    git fetch
    git checkout -b rebwithlibsharp origin/rebwithlibsharp

@mreineck , @crosset , @lpsinger, @PaulPrice please check this out before I merge it.