Tidy up.
Make the command registration use a default name if none is provided.

Credit for DefaultName method to @leavengood.
Switch to using the default names for commands.
Don't get the default name if explicitly specifiying a name.
Rename AllSelectionCommand to SelectAllCommand.
Don't expose RegisterWithDefault to the python.
Merge pull request #339 from erbridge/default_name

Default command name