add spawn script and make world empty by default

[dirk-thomas]

After launching an empty world:

roslaunch launch/empty_world.launch

the Iris / Plane models can be spawned using:

./script/spawn {iris,plane} <nsSuffix> [-x <X>] [-y <Y>]

[dirk-thomas]

The second commits adds a xml-model reference for validation. And based on the result I addressed the invalid value for the magnetic field.

[tfoote]

+1