Use fstring as part of futurize

Fstring is a PEP 448 backport of f-strings from python 3 to both python 2 and python 3

Installation is as easy as 
```
pip install fstring
```

Usage as easy as
```python
from fstring import fstring as f

a = 4
print(f("{a} is four!"))
```


Fstring should be used instead of string formatting or % formatting so developers could program in python 3 idioms.  