General cleanup

-Fix typos
-Add prerequisites to README.md
-Clean up everything.sh script
Add new hero images

-Update Centaur Warrunner, Sniper, and Tusk to new images
-Add Abaddon, Elder Titan, Skywrath, and Bristleback
Add new heroes

-Bristleback
-Elder Titan
-Skywrath Mage
-Abaddon
Add abbreviations for new heroes

-TC and ET go to Elder Titan
-LoA goes to Abaddon
Adjust Abbreviations, Fix BB description

-No longer says '$E' in Bristleback's guide
-Made 'TC' display Tauren Chieftan, and removed redundant Elder Titan
Actually fix Bristleback description
Update guides and changelog
Merge pull request #4 from robertsmieja/master

Bring Dota-2-Alt-Tab up to date