Improvements to NVD3 charts (axis labels & min bar width)

[mistercrunch]

Now the bar charts have a minimum bar width and overflow on the x axis. Also added support for axis labels on NVD3 charts.
@georgeke @williaster

[coveralls]

Coverage remained the same at 81.941% when pulling 943ff09 on mistercrunch:nvd3_plusplus into 65d9feb on airbnb:master.

[mistercrunch]

fixes #607

[georgeke]

while you're at this, perhaps we should also add a BooleanField for "reduceXTicks" for cleaning up things like this?

[mistercrunch]

Addressed comment. Mergin'