fixed bug when entering no author or more than one author
Several changes(not all are listed)
* fixed bug: creating a new empty company/location/category when entering none
* fixed bug: Nullpointer when selecting no location/category
* introduced BookDAO - replacemment for BookQuest?
* created credits dialog
* more bugfixed(can't remember them)
changed appearence of ChangeStatusDialog; fixed arcticle bugs in followedby-input and searchfield; made morebutton disabled if theres no following book
 fixec bug ignoring category and location