Run golint and fix as it suggests

Now running golint on every build
Only rebuild altas if needed

Now checks if sprite directory has changed before trying to pack the
atlas.
Added a flag to enable forcing a atlas rebuild even if it isn't needed.
Only construct output names in one place
Rename to be consistent with content
Move packing to its own package
Separate packing from Atlas struct

Previously packing was a method on the atlas, now it just uses the atlas
to give it the dimensions to pack to.
To do this Sprite is now an exported struct.