Gravity and drag in RGM

[mrkramerkss]

Lets add global gravity setting change (edit, project settings, physics 2d), and local changes to gravity and drag locally in the inspector for individual game objects.
It should probably go in the physics section.
Also, add a section on adding text in with the sprites section, include how to add fonts to Unity so that students can create text objects that are personalized. As of now they can only use Arial.

[DerekCresswell]

@Sanity-D says he'll deal with the Gravity and such.
The Text is big enough to be it's own issue seeing as it's a new section so I'll make a new issue. See #32