set the source location when calling YaST functions outside Ruby

fixes wrong location in y2log

(see https://github.com/yast/yast-ruby-bindings/issues/43)
better backtrace parsing
still log function reference as "YCPRef" for backward compatibility

using "YastRef" can break some tests
Merge pull request #50 from lslezak/ycpref_logging

Still log function reference as "YCPRef" for backward compatibility
Merge pull request #45 from lslezak/set_source_location

Set the source location when calling YaST functions outside Ruby
release version
fix test passing