Speed up deep copy of fields
Prevent constructing multiple nested serializers
Remove unnecessary iteration through _declared_fields
Pass instance to blog meta serializer in tests to make it fail

Expected failure. Need to fix #11
Fields are set upon setting the object
Fields are always updated whenever obj is set
Remove support for passing dicts for nested representations

Use Nested Serializer instead
Copy declared fields upon initialization

So don't have to deep copy everytime obj is set
_update_fields updates the fields attribute instead of returning fields
Cache serializer instance and reduce calls to _update_fields
Rename _intialize_fields -> _set_field_attrs
Better exception handling
Various cleanups

Remove unnecessary _marshal attribute
Default number field to float
If 'only' param is a field name, return early
Raise ValueError if fields or additional is not correct type
Remove declared_fields param from __get_opts_fields
If 'only' init param is specified, return early in _update_fields
Remove redundancy in _update fields

Also, rename __get_opts_fields -> __filter_fields
Move parameter validation code to SerializerOpts