fix negative number support

[vGimly]

there is a bug with negative numbers in Convert::Bencode - this patch add more complicated support for numbers.

negative numbers, like -1 sould be converted to number "i-1e", not to string "2:-1" like module does.

[m4ddav3]

I agree that this would be a good part to improve. However, I think this change is still imperfect.
If we use Scalar::Util::looks_like_number, it could differentiate between a value which is a number (1, 0, -1), or a string ("1", "0", "-1").