Support for implicit private members

Members with underscore prefixes are now implicitly private, which follows
common convention. See test case comments for rationale.
Added virtual kmask
Exposing keyword bit values and bitmasks

Available through property parser interface
Added support for implicit private members

Members with an underscore prefix are now implicitly private, which follows
conventions established in many object-oriented languages. This allows for a
concise definition style familiar to prototypal (and I suppose Ruby)
programmers.