New debugging features

[kevinfoley]

• New console command showMessage(message, [priority]): displays a message in the HUD, optionally with priority.
  
  
• New CVAR d_showData: when enabled, shows an extended version of what you get from the LADATA cheat (adds pitch, yaw, sector, tick)
  
• New CVAR d_enableDebug: controls debug rendering
• fixedPoint.h: added angleToFloatDegrees()
• console.cpp: fixed messages overflowing the console if they contained newline characters

[kevinfoley]

I used the same d_ prefix for the CVARs as you've used for other CVARS. I'm guessing that the d_ prefix stands for display, draw, or debug; in any case, it seems applicable. It might be helpful if you add a style guide for CVAR naming which defines the meaning of this prefix and other prefixes for future use (unless there's already one in the project that I've missed).