add c++ RBPotentialWrapper

and tests
add c++ angle axis RBPotentialWrapper

the c++ code is wrapped in cython.

the cython compiler couldn't find _pele.pyx, so I modified setup.py and
cythonize.py so pele/potentials/ could be passed to cython as an include
directory
add documentation to new c++ angle axis stuff

also do a bit of clean up