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
Ahh yes. I write lots of equipment drivers at work, where there are usually quite a bit of this kind of data e.g. something that explains how some returned code should be interpreted and therefore I'm in the habit of putting all those constants at the module level, because with large amounts it can make the class definition really big and tricky to read.

However, for this purpose that is not the case and it does belong better with the class.

It could however just as well be a class constant, so I agree with @stefankoegl's suggestion.