Add support for new additions to Aseprite spec for version 1.3

[TheCyberRonin]

Need to add support for the new additions in 1.3 of Aseprite

[jemc]

I don't know if these are all of the new 1.3 features, but PR #11 adds support for Tilesets and Tilemap layers, which is what I was looking for when I found this library.

[TheCyberRonin]

I'll take a look at the PR this evening!