Added virtual tech node for probe core passive antennas.

This patch resolves #52 by creating a dummy part in the tech tree that informs players that their probe cores now have a passive antenna. Naturally, the description will need to be changed if we ever allow some cores to get passive antennas at nodes other than Unmanned Tech.

![Appearance in tech tree](https://cloud.githubusercontent.com/assets/6760782/4100743/f35f2fd8-30b1-11e4-80e7-550bf3b21193.png)