Starfield fix and Milky Way

Fixed bug where more stars were generated at the poles.

Added 'Milky Way' band in sky, customisable with MILKY_WAY_WIDTH,
MILKY_WAY_INCLINATION and MILKY_WAY_RATIO consts in Generator.cpp.
Merge pull request #1142 from lordwaym/starfield-fix

Starfield fix and Milky Way