allow for arbitrary tilejson URLs
update template to match new config options
correctly receive backbone wrapped function argument

fixes
https://github.com/mapbox/tilemill-reference-layer/pull/9#issuecomment-28499850
don't override mapID with tilejson version
Mapbox Streets is no longer a default reference layer
add instructions for adding TileJSON to readme
clarify purpose of reference layer
a little more clarification in readme about mapID vs tilejson