implement finding of strong cooccurrences

but still have to play with the threshold
implement the calculation of max tf idf to use it for threshold calculation
Merge branch 'master' of github.com:nicolas-fricke/semmul2013-group1
start implementing usage of co occurring tags to find more pictures

but not yet implemented, only wrote a method, where it will be included
Merge branch 'master' of github.com:nicolas-fricke/semmul2013-group1