Merge pull request #29 from florianf/master

Option shownavigation=true/false that can be passed to ogCreateScene
removed wrong comment
 bugfix in cache
removed slow compiling shader

(angle problem, windows only)
adding new artwork
new code for empty terrain
added missing variable
new deploy version
improved stability for globe navigation
changed elevation limits
added new function: ogCreateNewContext
added tutorial for "ogCreateNewContext"
Hide/show logo fixed
Logo handling fixed