Allow functions r.js config by removing surrounding quotes
Whitelist generateSourceMaps, keepBuildDir, onModuleBundleComplete, skipDirOptimize options
Add logging to track precompile progress
Add source map support

Starting with r.js 2.1.2, experimental support for source maps was
added when using certain optimizers. In order for this to work,
preserveLicenseComments must be false.

See http://requirejs.org/docs/optimization.html#sourcemaps for details.

This change copies source maps from the target build dir to assets
and also copies all non-digested versions of all assets over as well
so that the source maps can reference them.