Add unit tests for RemoteStorage.defineModule

This adds a couple of very basic unit tests for RemoteStorage.defineModule.

The lines added to test/unit/remotestorage-suite.js make sure that
global.RemoteStorage gets defined, even if the same file was already
included in an earlier suite (this is a general problem with using
require inside jaribu tests)
Merge branch 'feature/680-defineModule_unit_tests'