fixed the bug 960

Hi I am Eduardo, I'm a student of UTFPR - Brazil, I was proposed in one of the subjects I am studying make some contributions to a free software project, so I was passed on this project. This is my first contribution, so follows the correction of a bug I found in the following link: http://sourceforge.net/p/jabref/bugs/960/. 

To fix the bug made ​​some changes to the system to validate the path provided by the user, and if the file does not exist it is created, checks were made for both Windows and for Linux. 

I am sending my pull-request. 

ps :. sorry my english