extended functionalities for skim module

Hi Christoph,
 this are the first batch of modifications. 
They will allow us to store in a structure tuple the collections produced 
during the typical analysis chain.

I have also prepared and committed a run macro which exploits these new
functionalities. Check it out here
https://github.com/dimatteo/MitMonoJet/blob/master/macros/runBoostedVntuple.C

An example of the output tuple is here:
/home/dimatteo/cms/root/boostedv_r12b-smu-j22-v1_noskim_0000_000.root