#307 Use WSAGetLastError and FormatMessage in evutil_socket_error_to_string for WIN32

[Alex Loukissas]

The current implementation leverages a static array of strings that holds the WinSock error code to error string mapping. The same thing can be done leveraging existing methods from the Win32 API, as briefly described here:

http://stackoverflow.com/questions/3400922/how-do-i-retrieve-an-error-string-from-wsagetlasterror