introducing levels

function new_editors now takes levels as thrid arguments
I should add this as optional argument from command lines soon.
levels for getopt

levels for getopt:
Usage:
$ python new_editor.py -o data/edits2011.json -n data/edits2012.json -l
1,2,3,4,5,6,7,8,9,10,50,100,500,1000
2012-01-01 - 2012-06-30
new editors with more than 1 edits:      53
new editors with more than 2 edits:      43
new editors with more than 3 edits:      40
new editors with more than 4 edits:      39
new editors with more than 5 edits:      36
new editors with more than 6 edits:      34
new editors with more than 7 edits:      33
new editors with more than 8 edits:      32
new editors with more than 9 edits:      31
new editors with more than 10 edits:     30
new editors with more than 50 edits:     11
new editors with more than 100 edits:    5
new editors with more than 500 edits:    2
new editors with more than 1000 edits:   2
Use Python list for levels handling in new_editors()

Our methods should take Python objects, the black magic used to obtain
the list of the levels from whatever should be kept outside.

Move the string splitting to getopt method.
Add levels handling to ArgumentParser