Add RosHostname to node executor.

Currently the App developer can access the RosMarterUri in a RosActivity. This PR adds a similar method to get the currently defined RosHostname.

To set the hostname the master chooser was modified to now list the available (and UP) network interfaces.

I made a simple modifications to the pubsub example to show how the RosHostname can be used in the applications.

This PR was mainly motivated because of the need to connect to an Android based robot through a VPN, and we needed to force the app to use the tunnel interface instead of the wlan one.

In order for this to work, the following PR in rosjava_core needs to be merged:
https://github.com/rosjava/rosjava_core/pull/177

I make this pull request to the hydro branch because it was stated in a mail that this is the one to use now, but it doesn't seem right. I guess this should be merged to the not yet created indigo-devel branch.