Allow remote repository to be located via non-http URI

[Alan Bateman]

The remote repository is currently limited to http. I've changed the 
implementation so that other URIs can now be used, in particular file 
URIs where the repository is on the file system. The webrev with the 
changes is here:

http://cr.openjdk.java.net/~alanb/jigsaw-localrepos/webrev/

Thanks,
Alan.