Fixed #9- can save Pointers to Objects

Fixed #9 by allowing ParseResources to be converted into Pointers and
back, such as when one object is set as an attribute of another. This
required:

- ParseResource now inherits Pointer
- All values are now converted to ParseType in the initialization of a
ParseResource
- The `_to_native` method of ParseResource was changed to `_to_dict`,
which allows `_to_native()` to construct a Pointer when one object is
an attribute of another.

Also:

- Added test coverage and example to README

- Fixed bug where many `from_native` methods used `self` instead of
`cls`

- `class_name` is converted to a string before being assigned to
`DerivedClass.__name__` (necessary because it is unicode when returned
from Parse)

- Changed TestUser to rely on parse_user.User instead of
parse_user.user.User