Add initial support for Windows

- Support by TUN/TAP driver from OpenVPN
- Native Windows API without emulation
- Client up and down sample .bat scripts

Limitations:
    * Only support MinGW, not support Cygwin
      due to the TUN/TAP driver
    * Daemonize not working, can only run in
      the foreground
    * Scripts can only be written in batch file
      format, bash not supported
Add compiling guide for Windows
Fix detection of old MinGW toolchains
Fix compilation with old MinGW toolchains
Sample scripts support for older Windows
Update README.md
Merge branch 'linusyang-windows' into dev
Merge remote-tracking branch 'origin/master'

Conflicts:
	README.md