Remove Map::source, add Tileset::source, change Image::source to PathBuf

Support object properties.

Added MIT license file

Closes #66

Fixes #72

There is only one image per Tile OR per Tileset. Added columns field.

Add layer id parsing

Add support for multiline string properties

Fix markdown formatting in CONTRIBUTORS file

update Zstd to 0.9

Reorganize crate

Move parse functions to its respective types, rename `Colour` for consistency with PropertyValue::ColorValue, add documentation to map members, add Map::source, misc improvements

Closes #99. ldtk seems to output tiled files with no newlines in the csv
data. Tiled itself does always seem to put the newlines in however, it
can open the ldtk files so we should too.

Implement Display for Orientation

Document that map dimensions are in tiles

And not, for example, in pixels

So that it can be more easily displayed to the user if needed