create a ipynbv3 json schema and a validator

cf http://json-schema.org/
http://tools.ietf.org/html/draft-zyp-json-schema-03

autogenerated from
http://www.jsonschema.net/index.html#
then tweeked.

This version makes all our notedooks in docs/example/notebooks valid
change prompt_numper type

prompt number type was number or string ('*', '&nbsp')
require it to be either number, or null
prompt_number : number -> integer
number -> integer
limit cell_level and nbformat(minor) values
require prompt number for code cell
import jsonpointer and schema from external
Add jsonschema liscence in external
update json pointer
update Jsonschema version
import argparse from external
Merge pull request #2396 from Carreau/json_schema

create a ipynbv3 json schema and a validator