Fix some compile warnings
Allow `new URL(String)` with nested JARs

Update JarFile to allow the custom registration of a JAR
`URLStreamHandler` that allows `jar:` URLs to be constructed from
Strings. This removes the previous requirement that all nested JAR URLs
be created with a 'context'.

To supported nested JARs the `java.protocol.handler.pkgs` system
property is changed so that our custom URLHandler is picked for 'jar'
protocols in preference to the Java default.

Fixes gh-269