FlxTilemap not clearing variables before loadMap

I've added a clearTilemap function that properly resets everything before every call to loadMap. I also set visible and active to false in the constructor, so that flixel doesn't attempt to draw a tilemap before it's been loaded.

Fixed FlixelCommunity/flixel#46, AdamAtomic/flixel#185
Fixed FlixelCommunity/flixel#34, AdamAtomic/flixel#197