Support r.js's baseUrl configuration

[yonilevy]

r.js allows configuring both an appDir and a baseUrl. Here there's optimizeSourceDir which is similar to appDir, but I can't find an equivalent to baseUrl. The lack of baseUrl means I have change my code to support brew, by prefixing every require module path with what would otherwise be my baseUrl (say js/).

Let me know if I'm missing something.