DATAJPA-407 - Fixes potential IndexOutOfBoundsException in ClasspathScanningPersistenceUnitPostProcessor.

So far, ClasspathScanningPersistenceUnitPostProcessor threw an IndexOutOfBoundsException on Windows as the OS-specific file separator doesn't match the forward slash used in the URI. This is fixed by modified scanForMappingFileLocations() to use '/' to build up the resource path.

Original pull request: #44.