Fixed YCP compatibility in Builtins.tointeger()

tointeger("") or tointeger("foo") should return nil, Ruby returns 0,
handle also the weird YCP cases like " 0x20" (-> 0) or " 020" (-> 20)
Merge pull request #42 from lslezak/tointeger_fix

Fixed YCP compatibility in Builtins.tointeger()