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.

Also add test for parsed image layers.
The test can be also used as an example off how one can get data from
the parsed map.

This commit also fixes a bug that meant the last cell of each row would
not be parsed if the tmx file was base64 encoded.