missing __init__.py and __future__ imports

I got some warnings from `pip install sympy`. This fixes 3 out of 4; I'm not sure what to do with

      File "/usr/local/lib/python2.7/dist-packages/sympy/mpmath/libmp/exec_py3.py", line 1
        exec_ = exec
                   ^
    SyntaxError: invalid syntax

but I guess it's harmless.