Implemented 'light near start' for EDFs

I've added a setting to EDFs for "light near start" (see issue #314). Points that are closer than this distance will not get lit by the EDF.
A side note: In Luxrender this feature is called "light near start" but in arnold it's "near attenuation", which might be a better term for it. What do you think?

Here are two images for comparison:
![no_light_near_start](https://f.cloud.github.com/assets/1567540/2490343/dd8429da-b1a3-11e3-95b8-34c84ad3e180.png)
![light_near_start](https://f.cloud.github.com/assets/1567540/2490342/d8fb0776-b1a3-11e3-8083-a31ca5c33486.png)