Users should not be allowed to change their uname to "guest"

[fgmart-zz]

Related to #115.

[derrell]

There are probably a few other special names that should be excluded. Off the top of my head, I can think of "admin" and "administrator" and maybe "superuser". There are probably a few others.

[derrell]

@mijoda to you. This should be simple. There is already a function that validates display names, so you just need to add a bit more sophistication to it. I suggest creating a static member which is an array of invalid display names, and ensuring that none of those (case-insensitive) is selected. (qx.lang.Array.contains(haystackArray, needleString) will be your friend.)

[cpuwhiz11]

Fix for this in karuto#8.