Better handling of signature method

[aezell]

Currently, flask-oauth is letting oauth2 do whatever it likes with regards to signature method. That means, it's mostly using HMAC_SHA1. However, some oauth servers will only accept PLAINTEXT as the signature method.

It's not too bad to set the signature method right after creating the remote_app, although it does mean accessing the _client which might not be totally obvious to some folks. It looks like this:

from flaskext.oauth import OAuth
from oauth2 import SignatureMethod_PLAINTEXT

oauth = OAuth()
my_api = oauth.remote_app('my_api',
    ...
)
my_api._client.set_signature_method(SignatureMethod_PLAINTEXT())

What doesn't work at all is using the authorized_handler decorator. When it calls make_client, it doesn't care about signature method. I had to add this around line 300 of oauth.py:

client.set_signature_method(oauth2.SignatureMethod_PLAINTEXT())

So, the issue is how to handle signature method better and allow for greater flexibility. Maybe if we just set it in the init() of OAuthRemoteApp(), we could just use it as a class attribute everywhere else. That assumes that a single remote_app always uses the same signature method, but from what I can see, that's a pretty safe assumption.