Update RInterface : correction of 'number of arguments' bug

Since R3.0 the number of arguments of External calls is checked for each call. sg has an unfixed number of argument. Fixing the number of arguments to -1 in the registration allows to disable that check.
Minor warning fix in the structured learning framework.
Comparison between signed and unsigned integers.
Merge pull request #1534 from jey1401/patch-1

Update RInterface : correction of 'number of arguments' bug
Merge pull request #1535 from iglesias/feature/fix_warning

Minor warning fix in the structured learning framework.