Parser.map adds empty Layer object when encountered.
Layer is parsed: Name, width, height.
UPdate README.
Parse the TMX "data" nodes.

Currently using ZlibDecoder, which only works on the server. Need to
decide: convert to a pure Dart library (would have to write my own), or
use TMX on the server and send a much more simplified version over the
wire.

Note: The dart 'zlib' library (the one on Github) is totally busted ATM, and
its breakage goes beyond simple name changes.

Note: implementation only supports encoding = base64 and compression =
zlib.

Note: Sending a simplified .yml file, gzipped by the server and decompressed
by the client, might be preferred over sending the default TMX, since the
default TMX would only be accurate at the time of server start -- once
players are moving around, the TMX is woefully out of date.