Added an allow_null option which adds the 'null' type to all properties in the schema.

This is useful when you need the schema to allow null values in the json being validated.
Identify all number values (both integers and floats) to be of type 'number'.

This is a hack for my own purposes.
Merge branch 'handle_integer_as_number'

* handle_integer_as_number:
  Identify all number values (both integers and floats) to be of type 'number'.