Nested sandboxes reuse `oasisURL` as default.

This is convenient for the common case that `oasisURL` needs to be defined 
exactly once.

Nested sandboxes can still define their own default by setting
`Oasis.config.oasisURL` or create sandboxes with a specific URL.


This makes fixing tildeio/conductor.js#20 trivial.

cc @Cyril-sf