Adds code to calculate optimal axis steps. Refs #13.
Removes camelCase style. Refs #13.
Fixes naming and usage of default_max_ticks. Refs #13.
Exports scale calculations to C. Refs #13.
Exports Nice_scale getters to C. Refs #13.

Works around https://github.com/Araq/Nimrod/issues/1189.
Documents task in changes log. Refs #13.
Merge branch 'feature/t13_graph_scale'