Issues with pandoc's self_contained mode

[jcheng5]

• Markers don't work, due to error: "Couldn't autodetect L.Icon.Default.imagePath, set it manually." This will also be a problem for the layers control if we add support for it.
• Doesn't work on IE8 at all (haven't tried other IEs). Actually now that I look at it, maybe self_contained isn't supported in IE8 in general due to poor support for data URIs? (Even a totally empty R Markdown document causes a JS error.)

Pandoc was also complaining during compile time about a URL that ended with #default#VML, I fixed that in d142e34. We will need to apply that patch every time we update Leaflet from upstream.

[jcheng5]

Markers fixed in 3ba6b26.

Confirmed with @jjallaire that self_contained doesn't work for IE8 (and possibly IE9).