Tool to skim events out of FSA ntuples

cc @mverzett 

Reads a list of
```
run:lumi:evt
```
from stdin and pulls out the rows in an FSA ntuple corresponding to those events.

Example:
```shell
cat /tmp/events_from_armin.txt | sed "s|.*Run_||" | sed "s|_LS_|:|" | sed "s|_Event_|:|" | scripts/skim_ntuple_events.py mmt/final/Ntuple test.root /scratch/mverzett/data/2013-Oct-08-8TeV/data_DoubleMuParked_Run2012*/*root
```

This part:
```
cat /tmp/events_from_armin.txt | sed "s|.*Run_||" | sed "s|_LS_|:|" | sed "s|_Event_|:|" | 
```
turns Armin's format into ``run:lumi:evt``.