Minor refactoring
Packing tilesets should walk all used tilesets, not only the last one

Previously the last loaded Map object was used to walk the tilesets and
pack them, now tilesets are being tracked consistently and enumerated
correctly.

Minor refactoring.

Add support for packing and loading multiple atlases in a single map,
adding a custom map property "atlas_<tilesetname>" to link a tileset to its
atlas.
Add new Tiled map loader with TextureAtlas support

Initial code from Justin Shapcott, thanks for sharing!

Port all changes from latest TmxMapLoader, load more map properties, check
for correct encoding
Minor refactoring
Add support for force-enabling custom texture filters
Add support for multiple TextureAtlases
Add null parameter check before use
Add new tests for TiledMapAtlas direct loading and AssetManager loading

Add resources (+preprocessed) to be able to run tests on checkout.

Note about resources: the "tiled-atlas-src" directory contains the source
used to build the "tiled-atlas-processed" resources: you can build the
preprocessor by running "ant -f build.xml gdx-tiled-preprocessor" in the
root libgdx/ directory.

Once built, run it against the source data, specifying an input and an
output directory:

<packer> <libgdx/.../tiled-atlas-src> <libgdx/.../tiled-atlas-processed>

Note that at this time the preprocessor will NOT overwrite the destination
folder if it already contains the same data, so make sure it's clean.