solution proposal for Issue #61

this should resolve #61

Note: documentation is not (yet) changed according to that. I first want to catch some feedback.

The aim with that idea is that Value instances can now be used independently from the configuration context as long as wished and properly configured (means the ```environ_name``` is given). In addition Values instances now cache their environment value to be accessible without calling ```setup_value``` once again.

some other features are now available as well:
```
            value = Value(environ_name='TEST', late_binding=True)
            self.assertEqual(type(value), Value)
            self.assertEqual(value.value, 'override')
            self.assertEqual(str(value), 'override')
            self.assertEqual('{0}'.format(value), 'override')
            self.assertEqual('%s' % value, 'override')
```