Fix loading of jar.

The process to get the file was incorrect if the jar was in a location containing RFC2396.

e.g. if a jar was in a folder containing spaces the file was attempted to be loaded not from
/foo/bar whizz/wibble.jar
but from
/for/mar%20whizz/wibble.jar
which ultimately is doomed to failure.

1) using a different method to get the jar
2) using a URI which handles the decoding of the escape sequences.
This fixes the issue by