Add DSO to example (broken for pip install) - #18

Hello,

This example was really helpful! I'm confused about how to correctly build/install a DSO that the pybind11 module needs to link. This PR adds a DSO (libarugula) to the example that the python module cmake_example links to (and exposes a function from, test_func).

This PR produces a working module in python setup.py develop or pip install -e . modes, but when running pip install . on the module, the installed version does not import because the RPATH on the installed cmake_example.so points to the temporary directory that pip used to compile the module. The module does however correctly install both cmake_example.so and libarugula.so (testing on linux). Does anyone have any suggestions?

> python -c "import cmake_example"        
Traceback (most recent call last):        
  File "<string>", line 1, in <module>
ImportError: libarugula.so: cannot open shared object file: No such file or directory

LDD info for the library:

> ldd cmake_example.so | grep arugula
        libarugula.so => not found

objdump:

> objdump -x cmake_example.so | grep RPATH
  RPATH                /scratch/tmp/pip-req-build-FfOtXz/build/lib.linux-x86_64-2.7

But all that said, setup.py did install both the cmake_example.so and the libarugula.so so I think its this is close:

> ls *.so
cmake_example.so* libarugula.so*  

And pip uninstall cmake_example also correctly wants to uninstall libarugula from site-packages:

DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7.
Uninstalling cmake-example-0.0.1:
  Would remove:
    .../lib/python2.7/site-packages/cmake_example-0.0.1.dist-info/*
    .../lib/python2.7/site-packages/cmake_example.so
    .../lib/python2.7/site-packages/libarugula.so

Thanks!