SRU implementation (#163)

I implemented SRU (#163), based on the SRU API of the GBV Verbundkatalog (http://gso.gbv.de/DB=2.1/), which by the way also uses another version of PICA (probably doesn't work with my PICA implementation).
I used JSoup's XML parser to parse the XML data, because XmlPullParser is quite painful if you need to parse such a large amount of tags and only need a small part of the data.
Not all the media properties are supported yet (only title, author name, year, description, ISBN and cover (fetched either from the URL in the XML response or, if not available, from Amazon using the ISBN)).
I think the GBV catalogue also doesn't show in the SRU response which libraries have the book, but when we add catalogues of single libraries using SRU, that's not needed anyway.