Change --cores to --jobs to be more consistent
Change cores to jobs in the code as well
Do not display "ocamlc: command not found" when initializing OPAM when no compiler is available
Remove the "-c" short-cut to "--comp" because "-comp" is then understood to "-c omp" which is quite ambiguous
Improve error reporting on 'opam init'.

* better error message in case the repository address is invalid
* better error message in case the compiler name is invalide

This should improve #357