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
Removed trailing whitespace.
Added usage of the threading API.
Added test for stack init.
Added threading tests to package.yml
Added a first simple implementation of threads.