Changes on the Java gui application

I was trying the Java application on my linux machine and found some issues and a minor improvement that I corrected in my branch. Copying the commit messages the changes addressed are the follwing:
    - Correction of a problem with the login process that prevented the login after one failed atempt. Added an option to the User and ModelUser getKey methods allowing the caller to force the generation of the key.
    - Correction of a problem in linux (Kubuntu) and OpenJDK 7 (7u71-2.5.3-0ubuntu0.14.04.1): The "Enter for copy" doesn't copy the password to the clipboard. Maybe there is a better way to fix the problem but the only way I found was to wait one second between the "Enter" action an the System.exit command.
    - It bothered me that the password was shown (and in big letters) every time the application is used so I added a checkbox (default "on") to mask most of the password (the application shows first and last characters of the password).