Support Basic authorization for access tokens

While developing an API that is using doorkeeper, I noticed that this popular flow was not supported for access tokens.

This would allow a user to authenticate themselves via:
```sh
curl -u "$TOKEN:" http://api.example.com/
```

I also added it to the generator and default config in commit 0fbd5f0. If that isn't desired, I can remove it and just have the  ```from_basic_authorization``` flow added as an optional addition for users to choose from.