Idea: Add "ping" command to visu plugin

This patch adds a simple `ping` command to the `visu` plugin. It can be used by the visualization to check if the backend is still reachable via the network. This enables the visualization to detect connection loss (e.g. network problems or SmartHome.py restarts).

I also implemented a simple check in the smartVISU to use this message to detect connection loss and try to reconnect automatically without reloading the page completely. This is especially handy if you want to use the visualization on a wall-mounted tablet which should be always up to day - even if the network was disconnected or SmartHome.py was not reachable for a while.

Without such a mechanism you can not guarantee that the data displayed in the visualization is always up to date. I also found a suggestion which is related to this in the smartVISU project [here](https://code.google.com/p/smartvisu/issues/detail?id=7).

Btw. why are the smartVISU guys still using subversion and Google Code instead of github.com? I also want to provide my patches to the smartVISU team without creating a Google Account - maybe there's something interesting for them.