Handle camelcase keys in JSON object

This addresses Issue #1

Add ExistingJSONKeys()

Add function to return a slice with the ids of all struct fields that will be set by the JSON object.

Handle "-" and "omitempty" JSON tags in struct definitions.

handle "-" and "omitempty" JSON tags in struct definition

Recognize struct definitions with JSON decoding instructions.

Add UnknownJSONKeys functionality.

Also minor refactoring of source files and documentation revisions.

Initial

Add release tag for reference purposes. (And because some people are using them.)