Instead of having to type:

p2diff file:///home/nboldt/tru/disco/jbtearlyaccesstarget/multiple/target/jbtearlyaccess-multiple.target.repo/ file:///home/nboldt/tru/disco/jbtearlyaccesstarget/multiple/target_FULL/jbtearlyaccess-multiple.target.repo/

I'd like to see it work for

p2diff target target2
or
p2diff site.zip site2.zip

So we need a few things:

a) find the nested repo in the specified folder (if not in `pwd` look in child folders for a p2 repo or repo zip)
b) better error reporting if the specified folders aren't repos or don't contain repos (right now p2diff does NOTHING if it doesn't like the URIs stated
c) if a local path exists, prepend file:// onto the file path so it behaves like a p2 URI
d) if a local repo zip found then prepend jar:file:// and append !/