The lazy-loading Scala.js env loads exported symbols eagerly.

Since we cannot decide when a top-level Scala.js symbol (class
or object) will be required when it is exported to JavaScript,
we have to load them eagerly.
Add tests for the simple features of @JSExport.

Complex corner-cases are not tested yet.
Fix #310: support @JSExport with qualified names for classes/objects.
Merge pull request #332 from sjrd/exports-qual-name

Support @JSExport with qualified names for classes/objects