Better library path management

[nrabinowitz]

There are a couple of options here, depending on whether we want to manage the paths in the build process as well (which would be nice).

• Breaking out the library path declarations into a separate module, a la registry.js, would allow me to get the paths with a Node script for the build process, the same way I do with the registry.
• Whether I take that approach or not, I need to offer a way for users (and module writers) to add a new library on the fly, pre-init - most likely with a registerLibrary() function.
• One more build note: In the build, I should probably ensure that common libraries - anything that could be used by multiple modules - don't get compiled into the module code. Probably this is as simple as taking the list of registered libraries and passing empty: for each path if/when we're building modules with r.js.