Rename demo folder and files.

Rename demo folder to pyCSGScript/exemples.
Rename demo files to have self explanatory names.
Update .gitignore
Add box.xml and box.obj to pyCSGScript/examples.
Change cwd when loading/saving files.

Now the cwd will match that of the current file, so that external references
can be written relatively to it.
Solve export bug for the color attribute.