MapReduce-lab: Solutions for Exercise 5

Signed-off-by: Enrico Canzonieri <enrico.canzonieri@eurecom.fr>
Signed-off-by: Antonio Uccio Verardi <antonio.verardi@eurecom.fr>
Added requested checks on cache file

Signed-off-by: Antonio Uccio Verardi <antonio.uccio.verardi@gmail.com>
Merge pull request #4 from poros/master

MapReduce-lab: Solutions for Exercise 5