ZeroNet fails to start after the ungraceful shutdown

After the system crash ZeroNet doesn't start any more:

```
[12:30:53] - OpenSSL loaded, version: 01000114F
[12:30:53] - Version: 0.3.6 r1098, Python 2.7.11 (default, May 6 2016, 08:39:38)
[GCC 4.2.1 Compatible FreeBSD Clang 3.4.1 (tags/RELEASE_34/dot1-final 208032)], Gevent: 1.1.1
[12:30:53] - Creating FileServer....
[12:30:53] TorManager Tor controller connect error: AssertionError: Authenticate error 515 Authentication failed: Password did not match HashedControlPassword value from configuration. Maybe you tried a plain text password? If so, the standard requires that you put it in double quotes.^M
in TorManager.py line 170
[12:30:53] - Creating UiServer....
Traceback (most recent call last):
File "/usr/local/share/zeronet/zeronet.py", line 16, in main
main.start()
File "/usr/local/share/zeronet/src/main.py", line 412, in start
actions.call(config.action, action_kwargs)
File "/usr/local/share/zeronet/src/main.py", line 115, in call
func(**kwargs)
File "/usr/local/share/zeronet/src/main.py", line 126, in main
ui_server = UiServer()
File "/usr/local/share/zeronet/src/Ui/UiServer.py", line 61, in __init__
self.sites = SiteManager.site_manager.list()
File "/usr/local/share/zeronet/src/Site/SiteManager.py", line 90, in list
self.load()
File "plugins/Zeroname/SiteManagerPlugin.py", line 17, in load
super(SiteManagerPlugin, self).load()
File "/usr/local/share/zeronet/src/Site/SiteManager.py", line 25, in load
for address in json.load(open("%s/sites.json" % config.data_dir)):
File "/usr/local/lib/python2.7/json/__init__.py", line 291, in load
**kw)
File "/usr/local/lib/python2.7/json/__init__.py", line 339, in loads
return _default_decoder.decode(s)
File "/usr/local/lib/python2.7/json/decoder.py", line 364, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "/usr/local/lib/python2.7/json/decoder.py", line 382, in raw_decode
raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded
- Starting ZeroNet...
```

/var/db/zeronet/data/sites.json is blank. File sites.json-new is valid, when I copied it to sites.json it works. This should happen automatically. Or, better you should use SQLite.

zeronet-0.3.6

从 src/Site/SiteManager.py 和 plugins/Zeroname/SiteManagerPlugin.py 开始，重点关注启动期间如何加载 sites.json。复现空 sites.json 导致的失败，并将其与有效的 sites.json-new 文件进行比较。完成的标准是：在此恢复场景之后，ZeroNet 无需手动替换文件即可启动；SQLite 方案则需要单独进行设计工作。

由索引模型根据 Issue 内容生成。