add missing log1p for windows

This now works with Windows (using Visual Studio (Express) compiler).
I tried it with windows8 and visual studio express 2012.
You need to fool python where to find the compiler though
``SET VS90COMNTOOLS=%VS110COMNTOOLS%``

Ref: http://stackoverflow.com/questions/2817869/error-unable-to-find-vcvarsall-bat/10558328#10558328

You may also need to do this:
http://stackoverflow.com/a/17065122/642882