Add support for variable arguments to PANIC

Now PANIC can be used with printf-like variable arguments, which is
useful to provide more detailed informations about a crash.
Add support for var args in default panic handler
Merge pull request #32 from antoinealb/panic-var-args

Add support for variable arguments to PANIC
Xmalloc: Fail if size == 0
Merge pull request #34 from antoinealb/malloc-fail-zero

Xmalloc: Fail if size == 0