Estimate AGM_BulletMass for unknown ammo.

For addons, which don't have the bullet mass correctly configured, I found out a pretty reliable way of estimating it from the hit and caliber values. This should get overheating working more or less realistically for all unsupported mods out there.

![image](https://cloud.githubusercontent.com/assets/7674951/5329209/f96ade9c-7d80-11e4-8a1c-e184485727d7.png)