Fix. `delete` method incorrect detect success.
Fix. Memcached ctor define global `err` variable.
Fix. `status` method detect error response.
Fix. `init_con` method use class instead of object to call `connect` method.
Change. Do not use `module` function.
Change. Use `lua-socket-async` to implement async IO.
Change. `stats` method could return info for specific key.
Fix. use `#` operator instead of `len` method.
Add. `on_key` callback method to change key before send to server.

```Lua
function mc:on_key(key) return 'namespace.' .. key end
```
Add. travis files.