Add support for native win32 platform

This pull request adds an initial support for building shadowsocks-libev as a native win32 app. It can be built on native Windows via MinGW (msys) or cross-compiled by using MinGW toolchain on other systems (tested on OS X). 

Building process is just the same as before: `./configure`, `make` and `make install`. All dependencies checking are added to the `configure.ac`. And this port doesn't break the compilation of other platforms, including Cygwin, so it should be safe to merge to the mainline.

However, at the moment, there are still some limits: only ss-local could be ported due to lacking win32 support of libasyncns. Besides, libev doesn't support IOCP but only supports select, so the performance will not be very ideal.

For testing, I have compiled a build, which is statically linked. Download is available [here](http://goo.gl/6twElU). If there is any bug, please let me know. Thanks.