New traversal first hacks
Even better traversals
Support nested traversals
Made traversals work with cursors

Also:
- renamed Traversal.list() to Traversal.get(),
- added .traversal() methods to Lens and Cursor
Merge branch 'better-traversals'
Updated traversals docs

- separate section
- updated interface
- documented nested traversals
Merge pull request #5 from Suor/master

Better traversals