Added hotkeys:

"WASD" - camera movement
"E" - upgrade tower
"Q" - the choice of targets
"F" - hellfire
"BACKSPACE" - selling tower
"1 2 3 4 5 6 7 8" - buy tower
"9" - airstrike
"+ - PgUp PgDown" - change the game speed
"ESC" - game menu
"Space" - pause