Default Sprite Behaviors: FollowerSprite

[pathunstrom]

In the interest of improving the on-boarding ramp for new users and learners, and by extension making our live demonstrations a little more impressive, we'd like to include some Sprites with basic control features.

For this ticket we are defining a FollowerSprite. Demoed simply at PyTexas this is a sprite that takes an another Sprite as it's target, and moves towards that sprite.

We'd like to provide a collection of movement strategies: various kinds of linear interpolation and a basic physics model with a velocity, acceleration and max velocity.

This all should be surfaced to users via parameters, users should only need to subclass FollowerSprite if they're adding to its behaviors.