fix custom manager inheritance.
change deserialization of None values
remove unused variables and imports
simplify to_python method.
fix deserialization of None values
remove managers patching on field initialization
add runtime managers patching via class_prepared signal
simplify comparison
Merge remote-tracking branch 'origin/master'

Conflicts:
	djmoney/models/fields.py
add isnull lookup support
remove managers patching on field initialization
add runtime managers patching via class_prepared signal
fix deserialization of None values
change list to iterator
fix typo
simplify comparisons and remove default value from dict.get method call
pep8, remove unused import
use currency_field_name to get field name
rename currency_field_name and move it to utils
Merge branch 'new_manager' into features_merge