fix THUCYDIDES-160 (no candidate steps found when path contains spaces)

The expression "new File(url.getFile())" is buggy because the result of
"url.getFile()" contains url-encoded characters (like "%20" instead of
space).
The fix is to use "new File(url.toURI())" instead, since that takes care
of decoding url-encoded characters.

See also: http://bugs.java.com/bugdatabase/view_bug.do?bug_id=4466485