start module index at 1 versus 0

Plays nicer with if (id) checks.
remove require preamble

browser-pack handle parent lookup now. It has to do it anyway to support
multi bundle
fix multi bundle support

add .external(filepath) which tells a bundle to expect a file in another
bundle and thus not load it into this one.

add .include(id | filepath) which will load a module or file into this
bundle as a non entry point. It will also expose it as a hash of the
full file path so other bundles can use it as an external

change .ignore(filepath) to only mean ignore file in this bundle