add colon to special characters that are removed from tags
Merge branch 'master' of github.com:nicolas-fricke/semmul2013-group1
Merge branch 'master' of github.com:nicolas-fricke/semmul2013-group1
Merge branch 'master' of github.com:nicolas-fricke/semmul2013-group1
only add each picture once for a synset in synset_filenames_dict