Added ".exe" extension to binary file on Windows

Running a binary on Windows does not work if it does not have the ".exe"
extension. 
Seriously, I'm not making this up! ;)

It wouldn't work:
```
[gin] listening on port 3000
2014/01/13 12:05:31 http: proxy error: dial tcp 127.0.0.1:3001: ConnectEx tcp: No connection could be made because the target machine actively refused it.
2014/01/13 12:05:32 http: proxy error: dial tcp 127.0.0.1:3001: ConnectEx tcp: No connection could be made because the target machine actively refused it.
2014/01/13 12:05:33 http: proxy error: dial tcp 127.0.0.1:3001: ConnectEx tcp: No connection could be made because the target machine actively refused it.
```
The app was not started because it didn't was a .exe, so nothing was listening on appPort.