Initial stable API release

This is the initial release of the API. Any documented APIs will try to remain stable from this point, and will go through a process of deprecation before change.

Differences from 0.1.2a2:

• Renamed the sparse_trees method to trees.
• Changed the ordering of records_out in the diffs iterator.
• Added documentation for more features in the TreeSequence class.
• Added set_mutations to allow user defined mutations.
• Added get_num_leaves and options to enable constant time implementation.