Branch:dec - adding return statement to all the decorator func

Bug
------
The client/token *getter* and *setters* in oauth provider 1 & 2 makes the original function not callable

Reproduce
-----
Steps:

    from . import oauth  #the oauth server object
    @oauth.grantgetter
    def load_request_token(token):
         print 'abc'
         return 1

    if __name__ == '__main__':
        load_request_token('cba')

Expectation:
Running the script will print out `abc`

Reality:
Won't be able to call load_request_token

Solution
----
Add `return f` to all the *getter* and *setter* in the providers

Reference
-----
Flask's *getter* decorator also return the f:

    @setupmethod
    def before_request(self, f):
        """Registers a function to run before each request."""
        self.before_request_funcs.setdefault(None, []).append(f)
        return f

https://github.com/mitsuhiko/flask/blob/master/flask/app.py#L1194

Test result
----

    (flask-oauthlib)davislx@vm-0:~/Github/flask-oauthlib$ nosetests                                                                                                                                              
    ..................................................................
    ----------------------------------------------------------------------
    Ran 66 tests in 194.731s
    OK