Project 2 - Game v0.2.3

Test disabling and restoring button functionality. Add disabled
attribute to btnWest and add removeAttribute("disabled") to btnGoNorth
function.

Move txtCommand.value reset from function goTxtCommand to function
updateHistory to ensure dirty text input is cleared by any valid
processed input. Otherwise, field could be left dirty if
txtCommand.value entered, then a btn[direction] clicked instead of
submitting text.

Move currentLocation to slightly different position in function
btnGo[direction] functions just for readability.