v0.1.6

Bug fixes and some changes:

• When nothing is mapped from the data, null is returned instead of an empty object
• Fields (on the mapping definition) with no path, are simply skipped
• Completed the set of path defs, and filter and transform pipelines, with 'From' and 'To' versions for normal and reverse mapping. Some have aliases without 'To' and 'From', to simplify the most basic definitions