Support drawing entities at desired rotation

* Set entity.angle in degrees.  Canvas will be rotated to that angle before calling draw.
Add ctx save/restore to fix renderer tests
Add tests for moving to view center for draw
Add tests to check entity draw rotation
Make centers actually align in test
Correct circ+rect collision test description
entities positioned with entity.center, not .pos

* `pos` meant top left.
* `center`, obviously, means center.
* This change was made to support sensible canvas rotation behaviour for entities with an entity.angle attribute.
* This change was made to make it totally clear that entities are positioned based on their center, not a corner.
Update docs on entity.center replacing .pos
Minify
Document `center` and `size` in creation eg
Document entity `angle` feature
Fix doc code sample indentation
v. 0.2.4
Add todo to gitignore
Make renderer.onScreen pass center, not top left
Minify
Doc formatting