mingw: make simavr compilable with MinGW

- patch makefiles to work with MinGW
- compile to run_avr.exe on windows, otherwise run_avr cannot be started from the normal windows command prompt
- add sim_network.h to encapsulate the network functions of different platforms
    (use winsock2 for Windows network functions)
- make network helper functions static
- force open() to use binary mode (on Windows default is text mode)
- do not change stderr on Windows
- exclude simduino example on Windows
- add MinGW README

Signed-off-by: Stephan Veigl <veigl@gmx.net>