Fix I2P support; add Windows .exe build script

Fixed I2P support to match the .bit spec.  Added a build script to generate a .exe file.  Both work for me but it would be great if someone can test.

To test I2P: ./nmcontrol.py dns getI2p_b32 anonymous-git-hosting.bit

To test the build script, install PyInstaller on Windows, and then run build_windows.bat.  The contents of the "dist" folder can then be transferred to a Windows machine without Python installed, and the resulting .exe file should work the same way that the .py file previously did.