Check if string is an integer or float

It's often helpful to be able to check whether a string contains a valid integer or float value, without having to resort to a full blown numericality validation.  

```
    "123".is_number?  => true
    "10.5".is_float?  => true
    "12.5".is_number? => true
    "12.x".is_number? => false
```