Always log file downloads and cache hits

Useful since the files can be stored in non-obvious locations (on OS X,
temporary directory is in /var/folders with per-user random subdir) and
may need to be manually removed for testing. Could consider multiple
logging levels instead of always logging these messages.. but they're
more useful to show than most of the other 'verbose' log messages.
Fix FileLocator path separator

File.separator is the separator used in subdirectory paths ('/'), whereas
File.pathSeparator is used in $PATH for separating multiple paths (':').
Using the wrong separator worked on OS X since the /var/folders subdir
could be created (":ss-cache") but failed on (Linux?) ci.md-5.net since
/tmp:ss-cache cannot be created (nor it should it be). With this change,
the path should be /tmp/ss-cache, as expected.