refactor into several npm modules

[tj]

to help people build their own builders if they dont want to use this one

• dep tree builder with lookup paths
• module definition with sourceurl option
• require related helpers
• url rewriting

[thehydroimpulse]

@visionmedia Also, it would be awesome if you provided hooks to implement custom module loaders. Right now (unless i'm missing something) i'm required to hack around the current builder and override a bunch of methods.

[jonathanong]

are you working on a tree builder yet? i was thinking of writing one that fixes #109

[tj]

yeah we'll need to separate the tree building to do a first pass and map all the requires so we can replace them without the aliases