Penalize for lack atmo shield - new feature

Traveling in the atmosphere implies damage to the hull.
Related to #2509

Flight from Earth to Cydonia/Mars (Autopilot - Sinonatrix without atmo-shield):
![screenshot-20140502-154017](https://cloud.githubusercontent.com/assets/1929111/2863156/5ef5de2e-d200-11e3-9b92-d02859fd09cd.png)

explosion at 0% of hull integrity (seems to be sufficient), not at temperature >= 100%