Add Ability to Promote Boolean Values

And here it is:

```python
from atom.api import Atom, Bool

class Test(Atom):
    b = Bool(strict=False)

t = Test() 

for truthy in [1, 0.5, -1.5, 'a', u'b']:
    t.b = truthy
    assert t.b
for falsey in [0, 0.0, '', u'']:
    t.b = falsey
    assert not t.b
```