Fix issue #22 (support HTML detail report for compiled CoffeeScript, etc)

Files with intermediate pre-processing (such as CoffeeScript) were failing on the default HTML reporter, as per issue #22. This commit adds an in-memory source cache to provide Istanbul with the exact JS code that was fed into the instrumenter (even when the original is CoffeeScript), so that full HTML detail reports can be generated on that JS.

Obviously full support for annotating CoffeeScript would be best implemented as part of Istanbul itself - but this at least fixes the exception and helps get some value of the detail report.