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
I think it makes sense to use the scope of the class, but I'd agree with @KennethNielsen that they should be constants, ie

```` python
class SoCo(_SocoSingletonBase):
    ...
    SEARCH_TRANSLATION = {'artists': 'A:ARTIST', ...}
````