Use a subpath for closed area interpolation.

This is more relevant for radial areas e.g. when using both stroke and
fill simultaneously on the same paths in a stacked radial area.
Add simple tests for closed radial areas.
Fix typo.
Slightly shorter.

This is also more future-compatible with *-closed interpolators!
Fix a bug when interpolating exponent notation.

We weren't properly detecting "1.e2" as a valid number.
Add sizzle selector dependency to node module build.
Fixed version specification for new sizzle dependency.
Add X-Requested-With: XMLHttpRequest header to xhr
Set X-Requested-With header for all cases
Fix background rect brush events for IE9.

I'm not sure pointer-events: all is necessary on the parent <g>, so I've
removed it assuming it's sufficient on the background <rect>.
Reinstate pointer-events: all on parent <g>.

This is required for the invisible resize handles.  Also added comment
about IE9 workaround.
An even better workaround for an IE9 bug.

It's as if IE9 is forgetting about the visibility: hidden rect, and
doing *something* to it jogs its memory.
Better workaround for IE9 bug.

I was mistaken in the previous commit (probably forgot to run make!)
Add another note about the workaround.

For consistency.
Use display: none for IE9 workaround.

Saves a line, too!
Use d3_identity instead of Object.
Add line.defined.
Add area.defined.
Better implementation of defined.

The previous implementation had a bug where the original index (i) was not
preserved if the input data array were split. The new implementation eschews the
previous approach of branching based on whether the x- and y-properties are
defined as functions or constants, opting for the simpler functor approach. The
x- and y-values are also coerced to numbers, as appropriate.
Merge branch 'area-closed' of https://github.com/jasondavies/d3 into line-defined