Fixes to run on unix/MATLAB R2013b

Dear Mark,

I read your recent blog post at undocumentedmatlab.com. Since I have converted most of my code projects to git, I started to explore JGit4MATLAB. I found the following changes necessary to run the code on Unix/MATLAB R2013b. Please review & apply.

* On unix, prefdir() is different from userpath(). According to R2013b docs, javaclasspath.txt is stored there.
* user.isEmpty and email.isEmpty does not work. getUserInfo returns MATLAB double arrays if information is not set. These arrays do not support dot notation for method calls.
* fprintf complained about not being able to print java strings, a conversion to char() is necessary.
* It might be good to know where a new javaclasspath.txt has been stored.

Best,

Volkmar