Added missing Return and Tab keycode support to library_glut.js

Return and Tab keys in GLUT are handled through the normal keyboard callbacks with ASCII codes (not a special key) and were missing. It's just a two-liner. A new side-effect with this patch is that Tab no longer switches focus from the emscripten canvas to the next web page element, but to me this is actually wanted behaviour, but of course feel free to reject the pull-request because of this :)

Also added myself to AUTHORS (seems to be required even for such a small change?).

PS: I'm still hanging with GLUT instead of SDL because it's so much slimmer, and I like the input through callbacks more then the general event-polling stuff in SDL. I was thinking about whether it might be a good idea to add a few super-slim emscripten_input_...() functions, which just wrap around JS input events, instead of trying to emulate some other API. Not sure whether this is a good idea though.