Added sky rendering.

Figures out limits of sky `super-sector' (i.e. the sector whose sky walls
should block rendering outside of it) and renders the sky as opaque walls using
a new sky shader.
Bumped version to 0.0.6.
Merge branch 'release/0.0.6'

Change log:
* Lighting effects (flickering, flashing etc.)
* Sky rendering.
Updated README to reflect current state.