Reduced the potency of carrying multiple light sources.

Modified how light levels are stored, and how items can adjust the player's luminosity.
Each light source on a player's person will still increase luminosity, but with diminishing returns.
This PR does not modify the brightness of any single object, but rather how they merge brightnesses.

Also fixed the bug where you'd lose more light than you should have when putting lit lights into containers.
Issues #1998 and #624

Fixes #1998 fixes #624

![2014-02-05_16-31-34](https://f.cloud.github.com/assets/1637936/2092830/acaf8d10-8eb0-11e3-82e7-5f4047032757.png)