1. A new property `contants` is added to the registry.
2. This property is just a reference to the `constants` Group.
3. This group is populated using the @group directive in the definitions file.
4. Minor changes to Group and System __getattr__
This change is fully backwards compatible:
```python
>>> ureg.speed_of_light
<Unit('speed_of_light')>
>>> 1 * ureg.speed_of_light
<Quantity(1, 'speed_of_light')>
>>> ureg.sys.imperial.speed_of_light
<Quantity(3.27857019e+08, 'yard / second')>
>>> ureg.constants.speed_of_light # if the current system is 'mks'
<Quantity(2.99792458e+08, 'meter / second')>
```
Close #1078