Fix browser builds to use common require / defineModule functions

When the core `orbit` module is defined, persist `define` as `global.Orbit.__defineModule__`
and `requireModule` as `global.Orbit.__requireModule__`.

Subsequent modules reuse these functions, which allows for cross-module dependency
lookups. It also tightens up the code so that the loader only needs to be defined once.
0.1.2