fields Float : returns a float

Was returning a string
marshal : add allow_null

You can chose if you want to return null fields or not
List : display_empty

Choose if you want to display empty list or not.
If true and the list is empty will return None
add a display_null

If you don't want to show key:null when you have a null value
Change dependencies

Not sure if the 1.3.0 is really needed.
This version is not included in Debian 7.
Merge branch 'display_null'

Conflicts:
	flask_restful/__init__.py
	flask_restful/fields.py