Add method to browse by string id

This is the result of the following discussion:

https://github.com/SoCo/SoCo/pull/192#issuecomment-53198218

Hopefully everyone is happy with this and it can be flagged for 0.9?

Thanks, Rob
Constants aren't constants in Python except by naming convention.

Class variables can be update like this:

```python
class A():

    MY_CONST_DICT = {'answer': 42}


class B(A):

    MY_CONST_DICT = A.MY_CONST_DICT.copy()
    MY_CONST_DICT.update({'question': 'the meaning...'})
    

b = B()
print b.MY_CONST_DICT
```