Attack speed is now upgradable

Syntax is the same as all other upgrade stats. This also allows attack
boosts to upgrade the attack speed (since they use the same code).
Animation speed now properly scales

That is, the increase in attack speed is the same as the increase in
animation speed. Previously, this was only the case for absolute
increases. Now percentage increases do so, as well.
Merge pull request #19 from MikeHoffert/feature-attack-boost

Attack boosts and upgrades now support upgrading attack speed