adapt comments in the command line arguments
add command line parameter for combined clustering

but they are not yet in usage
implement usage of -n and -i in combined clustering

you can use them to say how many jsons you want to read and where you want to start
add try catch to reading of jsons

because of one corrupted json in the folder 19
fix a bug

in get_small_image_url was an index error, therefore add a length check, but this could cause problems somewhere because the method returns None if there is no small url
Merge branch 'master' of github.com:nicolas-fricke/semmul2013-group1