Imtroduce the URLs module

The URLs module will contain URL manipulation utilities. For now, it
only contains a "join" method, which will start being used in the next
commit.
Use URLs.join instead of File.join to compose URLs

File.join uses File::SEPARATOR to join paths, which is not correct for
URLs. This commit changes code that joins URL parts to use URLs.join
instead. This method always uses "/" as a separator.

An alternative I considered was to use URI.join, but it's API is not
really suitable for our use cases.
Merge pull request #2 from SUSE/fix-url-joining

[Review] Request from 'dmajda' @ 'SUSE/pennyworth/fix-url-joining'