update rust-sdl2 temporarily to a PR branch so that it'll build w/ rust 0.9 master
update README to point at current rust ref commit
push Payload value into tile, get rid of weak Guid ref from Tile->World.payloads

this implies that downstream users looking for the option of top-down and bottom-up
search will need to keep their own indexes/references to Payloads down in the Tiles
.. or the Payloads just contains weak refs up to structures outside of World

eventually going to push the SpriteTiles out of Tile, as well, and the LoV
stuff out.. put it all in traits that downstream users will impl if they
want to use a World<T> with those parts of the library
add DrawableItem trait and integrate it into the basic draw_grid_tile routine,

will eventually push all of the SpriteTiles in Tile down into there, as well
and also probably pull offset/base out of the draw functions and just calc
them once per grid tile, having the grid work on the assumption that it'll
be rendering all sprites at the same upper-left coord (so that, if/when
multi-tile sprites come down, the tile rendering a non-standard sized
sprite can encapsulate that decision making.. also working towards pushing
the agent/entity/terrain stuff out of p2d and into the domain)
update rust-sdl2 tip ref
strip out unneeded serialize structs and build cleanup
remove rust-sdl2 from root of repo
add rust-mk
work on build infrastructure, clearing up weirdness w/ refactor and serialize/uuid moves