Execute on-jsload callback outside of core.async context.

Otherwise the handling of the go block catches the exception.
This is annoying for debugging because:

1) You must enable "Break on uncaught exceptions"
2) Stack traces are frequently lost
Merge pull request #53 from brandonbloom/no-go-catch

Execute on-jsload callback outside of core.async context.
made example changes
Merge branch 'master' into heads_up_wip
applying brandon's patch to wip