Implement new input system to make sure move forces are applied first, so things like jump can use move forces to modify actions.
Input handler now uses symbols instead of key IDs.
Process movement direction on button down instead of waiting for later in update loop to process all directions at once.

Reintroduces bug where the player can move faster on a diagonal.
Set velocity on in-air jump instead of adding to it to allow for rapid switching of direction through jumping.

Probably should modulate velocity based on move force or mass or both.
Set proper moments of inertia by default for objects with rectangle and circle colliders.
Implement resistive force for rotation to slow down spinning objects.
Add notes about in-air jump velocity.
Always apply movement force to move forward relative to character, and rotate body towards heading.
Create separate method to easily calculate the resitive torque, so that value can be easily accessed later.
Calculate resistive force and torque in Body so that those values can be used elsewhere if needed.
Flip order of g and u for Body#friction for similarity with #resistive_torque
Use Body#friction() to compute force needed to counteract friction.
Attempt to rotate body properly towards heading and then move forward.
Use dot product to correctly determine direction to rotatate while walking.
Properly compute friction based on both space component and body component.
Fix whitespace.
Merge old stashed stuff with equipment update commit.
Update rakefile to rid entity models directories of xml files.
Merge dangling commits from 'input_handler' with main branch.