[jsk_fetch_startup] Add network detector to switch services depending on the network

[708yamaguchi]

I added the node to detect whether fetch is connecting to the network.
We can check the network condition by rostopic echo /network/connected.

As an example of use, I'm going to use google speach_to_text when the network is on and use julius when it's off.

@mqcmd196

[k-okada]

So can we use #1285 to detect network status ?

[knorth55]

yes, we can us #1285, I think.
if we got 0 or no message from network_status.py, we can determine that the network is down.