Spawn unicorns on left and right screen sides
Change unicorn sprite angling
Work on unicorn movement
Improve unicorn behavior.

Unicorns now spawn on horizontal level edges, scan vertically for the
player, and charge horizontally. They can now also be eaten by a black
hole.
Merge branch 'improved-unicorn'
Merge pull request #32 from murphyslaw480/improved-unicorn

Improved unicorn