add spacing support to Tileset

[dmlary]

Updated Tileset to correctly adjust for spacing.

This only resolves the spacing portion of #22, not the padding; I don't have any tilesets with padding to verify.

I didn't add any additional test cases for this because I'm not sure this is how you'd like this support to be added. I didn't want to put a lot of effort into a new map & test cases until you verified this was the right approach. I can add them if you're ok with this fix.

[dmlary]

Hah, and I just saw there's a pending PR for this change already #18. Well, feel free to close this one as the other has support for both padding & spacing.