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
If they were constants, wouldn't that restrict the ability for a user to override the Soco class and add extra support for their custom app? (i.e. more mappings in this case) - This is what I have been doing in order to support more things before they are released.

Rob