Add instructions on enabling source maps to linked browser code.
replace barely-used lineForLine/includePositionInErrors with use of source maps.

Note that this is only triggered when coffeescript manages to output invalid JS,
which should be unlikely.

This does remove the feature where lines and columns were suppressed for parse
errors in the output of template compilation. but (a) that shouldn't happen,
and (b) we'll fix this by implementing source maps for spacebars.