Visible upgrades in tech tree

This is a port of MechJeb's system for visualizing tech upgrades to Procedural Fairings. This should alleviate some of the confusion about which technologies improve fairings.

Dummy parts will help users see where fairing upgrades appear in tech tree (see screenshot). Invalid values of `category` ensure that these parts will appear only in the tech tree, and never in the VAB. ![An example of a dummy part in the tech tree](https://cloud.githubusercontent.com/assets/6760782/3282481/e1ed5dde-f4f1-11e3-8222-7fa8760e5100.png)

One possible pitfall is KSP 0.24 compatibility. If 0.24 requires you to unlock parts separately after researching a technology, players may quickly realize that they do not actually need to unlock a dummy part to get the corresponding benefits.