[WIP] Use mpmath as a git submodule

This PR checkouts mpmath as a submodule in `sympy/external` and then links just the `mpmath` Python package into sympy.mpmath, so things should work as usual without any changes. I tried
```
python setup.py sdist
```
and this just copies the actual contents of mpmath into sympy/mpmath in the tarball (i.e. no symlink), so this is exactly how it was before.

I am just posting it here to see if Travis tests pass and to let people play with it. I am actually about -0.5 on using this approach, as git submodules are a bit annoying sometimes, but it does seem to do the job.