add USE_WIKITTEN_LOGO flag.

refer to https://github.com/victorstanciu/Wikitten/commit/b082e947d763f48405dfca507c8f8e85d546ed07
Merge pull request #6 from tamano/HideLogoOption

add USE_WIKITTEN_LOGO flag.
Merge branch 'master' of https://github.com/nicosomb/Wikitten