Demo camera refactoring

Demo camera refactoring
Changed camera binds, now it is mostly controlled by the standard movement keys (and the not so popular +move[up/down] cmds)
Fixed camera acceleration when moving in more than one dimension at the same time
Better behaviour when opposite direction keys are pressed at the same time
Restore chase_active value when disabling the camera or quitting the application

Better bot cvar descriptions

git-svn-id: svn://svn.icculus.org/nexuiz/trunk@5700 f962a42d-fe04-0410-a3ab-8c8b0445ebaa