Copy the sourceCodeLocation when we do a deep copy of the CssRulesetNode

When we do a deep copy of the CssTree, the source code location of the CssRulesetNode are not copied.
The side effect is that when a error is reported via a GssError, we receive a NullPointerException instead
of the error message.
Remove test.gss
Merge branch 'master' of github.com:google/closure-stylesheets into jd_fix_location
Merge branch 'master' of github.com:jDramaix/closure-stylesheet-gss into jd_fix_location
Merge pull request #1 from jDramaix/jd_fix_location

Copy the sourceCodeLocation when we do a deep copy of the CssRulesetNode