Improved Incremental Building; Added MapBuilder example

• Added MapBuilder -- simply write a function to map a source store document to a target store document. Supports automatic incremental building at the document level that is resilient to builder interruptions. Intended usage: subclass MapBuilder.
• Added GroupBuilder as an extension of MapBuilder -- group source docs and produce one target doc from each group.
• breaking change: Runner parameter num_workers renamed to max_workers to better reflect function.