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
Pleas use the more modern format function for formatting variables into strings: `search_uri = "#{0}{1}".format(search, idstring)`