make Readable for boolean fields more user-friendly

As far as I can see, if using `Readable` and `get_readable`, boolean fields are represented as `f` and `t` (False and True). It would be good if it was possible to represent these in a more user-friendly way - Maybe even make the default similar to Python's string representation of booleans:

```
In [1]: str(True)
Out[1]: 'True'

In [2]: str(False)
Out[2]: 'False'

```