Remove duplicate Nifty resources in assets folder.

For the android module of nifty-examples-libdx, Nifty's resources must
be copied to the nifty-examples-libgdx/assets folder. They were manually
copied over, but maintaining two copies of all of Nifty's resources
proved to be very difficult.

For now, android module will remain broken until I can figure out how to
use Maven to copy all of Nifty's resources on-the-fly into the assets
folder (and by extension, into the .apk).
Add LibGDX mention / logo in main example credits.