Untrack automatically generated files
Make item classes serializables.
New. Unit tests for atlantis.gamedata.item module.
New. Documentation file for atlantis.gamedata.item module.
Add links to the following modules documentation:

   atlantis.gamedata.item
   atlantis.gamedata.gamedata
   atlantis.gamedata.map
Rename the file, as now an index.rst file will be the index for the
whole package.

Add dump and load functions for dict.

Improve module documentation.
New documentation index file for atlantis.helpers package.
New documentation file for atlantis.helpers.comparable module.
New unit tests for atlantis.helpers.comparable module.
Link to atlantis.helpers package for documentation instead of individual
modules in the package.