Cornell Box scene as a test 
Will get rid of those bright dots later. But finally the light source looks right.
Plans for next:
1) Spectral Rendering
There is already some support for it built in PBRT. Just needs to figure it out how to implement it.
2) Daylight Sky Model
Preetham, Shirley and Smits'
physically based sky model. Both Maxwell Render and Indigo use this model. Very realistic skylight especially when combined with spectral rendering !!
3) Distributed Rendering
To improve the render time, it has to be parallelized, and jobs can be distributed to some remote machines, e.g., a render farm.
An update for the Sponza scene(render time ~6 or 7 hours) 