Update Notes API to support key=pair additions and lookup

Update Notes API to support key=pair additions and lookup

* Adds a Notes::const_iterator definition to match Notes::iterator

* Adds a Notes::findByName(note-name) method to locate notes given their
  textual key / name string.

* Adds a Notes::add(key, value) method to insert key and value as a
  single operation. No more need for callers to be using Note::Pointer
  or be aware of the Note:: API just to insert the value.

TODO:

* add "key=value" syntax parser to Notes. The current parse() method
  onlyl accepts MiME header syntax (line- and whitespace- oriented).

* add ostream operator to dump the notes list content to a stream.