basic note iterator implementation

* git_note_iterator_new() - create a new note iterator
* git_note_next() - retrieves the next item of the iterator
use git_note_iterator type instead of non-public git_iterator one
fixed minor issues with new note iterator

* fixed style issues
* use new iterator functions for git_note_foreach()
added missing free for git_note in clar tests
Merge pull request #1396 from cholin/features/note-iterator

[RFC] basic note iterator implementation