built on windows

The ```mman``` for Windows implementation is from https://code.google.com/p/mman-win32/.

And now RcppAnnoy passed the ```R CMD check``` on windows (http://win-builder.r-project.org/XXJ1PZFHgu7l/00check.log).

I have used this ``mman`` implementation  when porting software to Windows. It works well although the efficiency may not be that good.