Merge pull request #93 from thinred/develop

provide free versions of skyboxes
Updated ChangeLog for the last few years' commits
Version bump to 0.5.1
Merge branch 'release/0.5.1' into develop
bumped version number to 0.5.1
replaced non-free images with free equivalents
Merge branch 'hotfix-0.5.1' into develop

Conflicts:
	ChangeLog
	src/gravit.h