Add croak_sv

This PR (tries) to add support for ```croak_sv```. Not sure if this is the correct place to put it and/or the tests that are required (or how to write them). Verified that it does work for perl 5.10 and 5.12, see:

https://travis-ci.org/ghedo/p5-Git-Raw/builds/25368088
https://github.com/ghedo/p5-Git-Raw/compare/4286a050708d...57d263003b81