Implement float is_integer method

## python
```
>>> a = 10.0
>>> type(a)
<class 'float'>
>>> a.is_integer()
True
```

## gpython
```
>>> a = 10.0
>>> type(a)
<class 'float'>
>>> a.is_integer()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 FIXME line of source goes here
AttributeError: "'float' has no attribute 'is_integer'"
```

`is_integer()` method does not exist.