v0.6.0

• Add selection.raise and selection.lower.
• Selections are now bilevel, as in D3 3.x, not arbitrary hierarchies.
• Selections functions now take the current datum d, index i and group nodes as arguments.
• Change selection.enter and selection.exit to reset enter and exit after access.
• Change selection.call to no longer set the this context.
• Fix d3.namespace to string-coerce.
• Fix selection.classed when element.getAttribute returns null.
• Fix enter.select to not clear the enter selection.
• Fix enter nodes for element.querySelectorAll.
• The requote function is no longer exported.
• Selection methods have been optimized.