general cleanup

[sbarthelemy]

There several small cleanups that could be done but would break compatibility.
Better to discuss them here and perform all of them at the same time.

1. rename World.iterbodies to World.bodies
2. remove World.getbodies (doing NamedObjectsList(World.bodies)) is easy enough
3. remove robots/simpleshapes.py. They are only used for tests and not flexible enough for other uses
4. change the extension of the doc files from rst to txt