Change buildModuleUrl to always return an absolute URL.

Fixes #779.
Simplify some code now that buildModuleUrl always returns absolute URLs.
Add a basic test for buildModuleUrl in require.toUrl mode, which is what the specs run under.
Make sure the URL returned by buildModuleUrl actually loads.
Merge pull request #780 from AnalyticalGraphicsInc/fixWorkerCreationFromBlobUrls

Change buildModuleUrl to always return an absolute URL.