Can not abort "python zeronet.py siteDownload" command

### Step 1: Please describe your environment

Version: 0.6.1 r3221, Python 2.7.5 (default, Aug 4 2017, 00:39:18)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-16)], Gevent: 1.0
OS: Linux 64 bit

### Step 2: Describe the problem:

If site does not have any peers, the "python zeronet.py siteDownload" command gets stuck and i can not abort it:

> [user@localhost ZeroNet]# python zeronet.py siteDownload addresshere
> - Starting ZeroNet...
> - OpenSSL loaded, version: 01000207F
> - Version: 0.6.1 r3221, Python 2.7.5 (default, Aug 4 2017, 00:39:18)
> [GCC 4.8.5 20150623 (Red Hat 4.8.5-16)], Gevent: 1.0
> - Opening a simple connection server
> TorManager Connecting to Tor Controller 127.0.0.1:9051
> TorManager Tor controller connect error: error: [Errno 111] Connection refused in TorManager.py line 170 > socket.py line 342
> TorManager Starting self-bundled Tor, due to Tor proxy port 127.0.0.1:9050 check error: No connection
> Site:1KR7WW..hMPK Content.json not exist: data/addresshere/content.json
> Announcing...
> Downloading...
> ConnServer Internet offline
> ^CShutting down...
> - Unhandled exception
> None
> KeyboardInterrupt
> ^CShutting down...
> - Unhandled exception
> None
> KeyboardInterrupt
> exit
> ^Ad^CShutting down...
> - Unhandled exception
> None
> KeyboardInterrupt

Keyboard interrupt means i did abort command Ctrl+C in my console (Putty SSH client)

从入口点 `python zeronet.py siteDownload` 开始，使用一个没有 peer 的站点重现该命令。跟踪 Ctrl+C 后的关闭路径，并验证进程能够干净地退出，不会反复输出 KeyboardInterrupt 或未处理异常信息。

由索引模型根据 Issue 内容生成。