Add a FloatRange

I'm not sure what I'm missing, it compiles but I get this error:

```python
>>> from atom.api import FloatRange
>>> from atom.api import FloatRange, Atom
>>> class Test(Atom):
...     r=FloatRange()
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in Test
  File "c:\users\silvester\workspace\atom\atom\scalars.py", line 133, in __init__
    self.set_validate_mode(Validate.FloatRange, (low, high))
AttributeError: type object 'Validate' has no attribute 'FloatRange'
> c:\users\silvester\workspace\atom\atom\scalars.py(133)__init__()
-> self.set_validate_mode(Validate.FloatRange, (low, high))
```