doc/make.py only works for development build
doc/make.py contains the statement `os.environ['PYTHONPATH'] = '..'`, which means it will only work with an inplace build. I need to be able to contribute to pandas documentation using an installed version of pandas. When I comment out this line, I am able to build the html documentation from source. 

See this [SO question](http://stackoverflow.com/questions/30807270) for background and details.
