Async @requires

[rofinn]

Improve package load time by making listenmod asynchronous with (@schedule). Closes #39.

[MikeInnes]

Do we know how this will behave if, for example, someone does using A, B in a script and then immediately depends on interop between them? Can we guarantee that the @require block won't be delayed and break stuff?

[rofinn]

I'm pretty sure that will break. I'm not really sure what the best solution is.